diff --git a/geoh5py/ui_json/annotations.py b/geoh5py/ui_json/annotations.py
index 9de8c97b0..3cc38f48f 100644
--- a/geoh5py/ui_json/annotations.py
+++ b/geoh5py/ui_json/annotations.py
@@ -27,7 +27,7 @@
 from geoh5py.data import DataAssociationEnum, DataTypeEnum
 from geoh5py.groups import Group
 from geoh5py.objects import ObjectBase
-from geoh5py.shared.utils import enum_name_to_str, stringify
+from geoh5py.shared.utils import enum_name_to_str, none2str, str2none, stringify
 from geoh5py.shared.validators import (
     to_class,
     to_list,
@@ -80,6 +80,12 @@ def deprecate(value, info):
     PlainSerializer(types_to_string, when_used="json"),
 ]
 
+OptionalPath = Annotated[
+    Path | None,
+    BeforeValidator(str2none),
+    PlainSerializer(none2str),
+]
+
 OptionalUUID = Annotated[
     UUID | None,
     BeforeValidator(optional_uuid_mapper),
diff --git a/geoh5py/ui_json/forms.py b/geoh5py/ui_json/forms.py
index 6897b3f66..c5f3d7cdc 100644
--- a/geoh5py/ui_json/forms.py
+++ b/geoh5py/ui_json/forms.py
@@ -90,10 +90,10 @@ class BaseForm(BaseModel):
 
     model_config = ConfigDict(
         extra="allow",
-        frozen=True,
         populate_by_name=True,
         loc_by_alias=True,
         alias_generator=to_camel,
+        validate_assignment=True,
     )
 
     label: str
@@ -129,8 +129,6 @@ class and lastly fall back on type checking the value field of the form.
         fields to avoid false positives.
 
         :param data: Form data.
-        :param form_types: Pre-compute all the base classes to check against.
-        :param indicators: Pre-compute the indicator attributes for each subclass.
         """
 
         data = {to_snake(k): v for k, v in data.items()}
@@ -154,6 +152,13 @@ def flatten(self):
     def validate_data(self, params: dict[str, Any]):
         """Validate the form data."""
 
+    def set_value(self, value: Any):
+        """Set the form value."""
+        self.value = value
+
+        if "optional" in self.model_fields_set:
+            self.enabled = self.value is not None
+
 
 class StringForm(BaseForm):
     """
@@ -556,17 +561,31 @@ def property_if_not_is_value(self):
             and not isinstance(self.property, UUID)  # pylint: disable=unsupported-membership-test
         ):
             raise ValueError("A property must be provided if is_value is used.")
+
         return self
 
     def flatten(self) -> UUID | float | int | None:
         """Returns the data for the form."""
-        if (
-            "is_value" in self.model_fields_set  # pylint: disable=unsupported-membership-test
-            and not self.is_value
-        ):
+        if "is_value" in self.model_fields_set and not self.is_value:
             return self.property
         return self.value
 
+    def set_value(self, value: Any):
+        """Set the form value."""
+        try:
+            self.value = value
+            self.is_value = True
+        except ValidationError:
+            if value is not None:
+                self.property = value
+                self.is_value = False
+            else:
+                self.is_value = True
+                self.property = None
+
+        if "optional" in self.model_fields_set:
+            self.enabled = value is not None
+
 
 class MultiSelectDataForm(DataFormMixin, BaseForm):
     """
diff --git a/geoh5py/ui_json/ui_json.py b/geoh5py/ui_json/ui_json.py
index 17dc3bfd7..450bca890 100644
--- a/geoh5py/ui_json/ui_json.py
+++ b/geoh5py/ui_json/ui_json.py
@@ -23,14 +23,12 @@
 import json
 import logging
 from pathlib import Path
-from typing import Annotated, Any
+from typing import Any
 from uuid import UUID
 
 from pydantic import (
     BaseModel,
-    BeforeValidator,
     ConfigDict,
-    PlainSerializer,
     create_model,
     field_validator,
 )
@@ -39,24 +37,18 @@
 from geoh5py.groups import PropertyGroup, UIJsonGroup
 from geoh5py.shared import Entity
 from geoh5py.shared.utils import (
+    as_str_if_uuid,
+    dict_mapper,
+    entity2uuid,
     fetch_active_workspace,
-    none2str,
-    str2none,
-    str2uuid,
-    stringify,
 )
+from geoh5py.ui_json.annotations import OptionalPath
 from geoh5py.ui_json.forms import BaseForm
 from geoh5py.ui_json.validation import ErrorPool, UIJsonError, get_validations
 
 
 logger = logging.getLogger(__name__)
 
-OptionalPath = Annotated[
-    Path | None,
-    BeforeValidator(str2none),
-    PlainSerializer(none2str),
-]
-
 
 class BaseUIJson(BaseModel):
     """
@@ -259,7 +251,7 @@ def flatten(self, skip_disabled=False, active_only=False) -> dict[str, Any]:
 
         return data
 
-    def fill(self, copy: bool = False, **kwargs) -> BaseUIJson:
+    def set_values(self, copy: bool = False, **kwargs) -> BaseUIJson:
         """
         Fill the UIJson with new values.
 
@@ -269,38 +261,20 @@ def fill(self, copy: bool = False, **kwargs) -> BaseUIJson:
 
         :return: A new UIJson object with the updated values.
         """
-        temp_properties = {}
-        for key, form in dict(self).items():
-            if not isinstance(form, BaseForm):
-                if key in kwargs:
-                    if not isinstance(kwargs[key], str):
-                        raise TypeError(
-                            "Only string values can be updated for non-form fields. "
-                        )
-                    temp_properties[key] = kwargs[key]
-                continue
-
-            updates: dict[str, Any] = {}
-
-            # if a value has no default value, set enabled to false
-            if not bool(form.value) if form.value != [""] else False:
-                updates["enabled"] = False
-
-            if key in kwargs:
-                updates["value"] = str2uuid(stringify(kwargs[key]))
-                updates["enabled"] = True
-
-            if updates:
-                temp_properties[key] = form.model_copy(update=updates)
-
-        updated_model = self.model_copy(update=temp_properties)
+        if copy:
+            uijson = self.model_copy(deep=True)
+        else:
+            uijson = self
 
-        if not copy:
-            for field_name in type(self).model_fields:
-                setattr(self, field_name, getattr(updated_model, field_name))
-            return self
+        demotion = [entity2uuid, as_str_if_uuid]
+        for key, value in kwargs.items():
+            form = getattr(uijson, key, None)
+            if isinstance(form, BaseForm):
+                form.set_value(value)
+            else:
+                setattr(uijson, key, dict_mapper(value, demotion))
 
-        return updated_model
+        return uijson
 
     def to_params(self, workspace: Workspace | None = None) -> dict[str, Any]:
         """
diff --git a/recipe.yaml b/recipe.yaml
index 613450daa..55952ceec 100644
--- a/recipe.yaml
+++ b/recipe.yaml
@@ -2,7 +2,7 @@ schema_version: 1
 
 context:
   name: "geoh5py"
-  version: "0.0.0.dev0"  # This will be replaced by the actual version in the build process
+  version: "0.12.1rc2.dev279+3df1d110"  # This will be replaced by the actual version in the build process
   python_min: "3.12"
   module_name: ${{ name|lower|replace("-", "_") }}
 
diff --git a/tests/ui_json/forms_test.py b/tests/ui_json/forms_test.py
index b22384973..9def33ce4 100644
--- a/tests/ui_json/forms_test.py
+++ b/tests/ui_json/forms_test.py
@@ -97,12 +97,6 @@ def test_base_form_config_extra(sample_form):
     assert form.model_extra == {"extra": "stuff"}
 
 
-def test_base_form_config_frozen(sample_form):
-    form = sample_form(label="name", value="test")
-    with pytest.raises(ValidationError, match="Instance is frozen"):
-        form.label = "new"
-
-
 def test_base_form_config_alias(sample_form):
     form = sample_form(
         label="name",
@@ -515,6 +509,32 @@ def test_data_or_value_form():
             property="",
         )
 
+    form.set_value(None)
+    assert form.is_value
+
+    optional_form = DataOrValueForm(
+        label="name",
+        value=1.0,
+        parent="my_param",
+        association="Vertex",
+        data_type="Float",
+        is_value=True,
+        property="",
+        optional=True,
+    )
+    assert optional_form.enabled
+    optional_form.set_value(None)
+    assert optional_form.is_value
+    assert not optional_form.enabled
+
+    optional_form.set_value(data_uid)
+    assert not optional_form.is_value
+    assert optional_form.property == uuid.UUID(data_uid)
+    assert optional_form.enabled
+
+    form.set_value(2)
+    assert form.value == 2
+
 
 def test_multichoice_data_form():
     data_uid_1 = str(uuid.uuid4())
diff --git a/tests/ui_json/uijson_test.py b/tests/ui_json/uijson_test.py
index cd47c5ea9..82fcfc1d6 100644
--- a/tests/ui_json/uijson_test.py
+++ b/tests/ui_json/uijson_test.py
@@ -617,7 +617,7 @@ class MyUIJson(BaseUIJson):
             "my_int_parameter": {"label": "b", "value": 1},
         },
     )
-    result = uijson.fill(my_string_parameter="updated")
+    result = uijson.set_values(my_string_parameter="updated")
 
     assert result is uijson
     assert uijson.my_string_parameter.value == "updated"
@@ -635,31 +635,15 @@ class MyUIJson(BaseUIJson):
         uijson=MyUIJson,
         data={"my_string_parameter": {"label": "a", "value": "original"}},
     )
-    copy = uijson.fill(copy=True, my_string_parameter="updated", title="ok")
+    copy = uijson.set_values(copy=True, my_string_parameter="updated", title="ok")
 
     assert copy is not uijson
     assert copy.my_string_parameter.value == "updated"
     assert uijson.my_string_parameter.value == "original"
     assert copy.title == "ok"
 
-    with pytest.raises(TypeError, match="Only string"):
-        _ = uijson.fill(copy=True, my_string_parameter="updated", title=666)
-
-
-def test_fill_disables_forms_with_falsy_value(tmp_path):
-    ws = Workspace(tmp_path / "test.geoh5")
-
-    class MyUIJson(BaseUIJson):
-        my_zero_param: FloatForm
-
-    uijson = generate_test_uijson(
-        ws,
-        uijson=MyUIJson,
-        data={"my_zero_param": {"label": "a", "value": 0.0}},
-    )
-    uijson.fill()
-
-    assert uijson.my_zero_param.enabled is False
+    with pytest.raises(ValidationError):
+        _ = uijson.set_values(copy=True, my_string_parameter="updated", title=666)
 
 
 def test_fill_truthy_value_leaves_updates_empty(tmp_path):
@@ -675,7 +659,7 @@ class MyUIJson(BaseUIJson):
         data={"my_param": {"label": "a", "value": 3.14}},
     )
     original_enabled = uijson.my_param.enabled
-    uijson.fill()
+    uijson.set_values()
 
     assert uijson.my_param.enabled == original_enabled
     assert uijson.my_param.value == 3.14
@@ -690,9 +674,11 @@ class MyUIJson(BaseUIJson):
     uijson = generate_test_uijson(
         ws,
         uijson=MyUIJson,
-        data={"my_param": {"label": "a", "value": 0.0, "enabled": False}},
+        data={
+            "my_param": {"label": "a", "value": 0.0, "enabled": False, "optional": True}
+        },
     )
-    uijson.fill(my_param=5.0)
+    uijson.set_values(my_param=5.0)
 
     assert uijson.my_param.enabled is True
     assert uijson.my_param.value == 5.0
@@ -717,7 +703,7 @@ class MyUIJson(BaseUIJson):
             }
         },
     )
-    uijson.fill(my_object_parameter=pts2.uid)
+    uijson.set_values(my_object_parameter=pts2.uid)
 
     assert uijson.my_object_parameter.value == pts2.uid