Use a variable with a subset of "Type" field options (let's say " ,Item,Fixed Asset") and show it in the page of "Type" field. In the new control OnValidate trigger manage the field assignment like:
CASE NewType OF
NewType::Item: VALIDATE(Type,Type::Item);
NewType::"Fixed Asset": VALIDATE(Type,Type::"Fixed Asset");
END;
This because the options of the variable do not correspond anymore to the options of the table field.