Emit oldStored based on the old column in CSharpMigrationOperationGen…

src/EFCore.Design/Migrations/Design/CSharpMigrationOperationGenerator.cs

@@ -640,7 +640,7 @@ protected virtual void Generate(AlterColumnOperation operation, IndentedStringBu
                     .Append("oldComputedColumnSql: ")
                     .Append(Code.Literal(operation.OldColumn.ComputedColumnSql));
 
-                if (operation.IsStored != null)
+                if (operation.OldColumn.IsStored != null)
                 {
                     builder
                         .AppendLine(",")

test/EFCore.Design.Tests/Migrations/Design/CSharpMigrationOperationGeneratorTest.cs

@@ -633,6 +633,37 @@ public void AlterColumnOperation_all_args()
                 Assert.Equal("Some Collation", o.OldColumn.Collation);
             });
 
+    [Fact]
+    public void AlterColumnOperation_computed_to_non_computed_preserves_oldStored()
+        => Test(
+            new AlterColumnOperation
+            {
+                Name = "Id",
+                Table = "Post",
+                ClrType = typeof(int),
+                OldColumn =
+                {
+                    ComputedColumnSql = "1",
+                    IsStored = true
+                }
+            },
+            """
+mb.AlterColumn<int>(
+    name: "Id",
+    table: "Post",
+    nullable: false,
+    oldComputedColumnSql: "1",
+    oldStored: true);
+""",
+            o =>
+            {
+                Assert.Equal("Id", o.Name);
+                Assert.Equal("Post", o.Table);
+                Assert.Equal(typeof(int), o.ClrType);
+                Assert.Equal("1", o.OldColumn.ComputedColumnSql);
+                Assert.True(o.OldColumn.IsStored);
+            });
+
     [Fact]
     public void AlterColumnOperation_DefaultValueSql()
         => Test(