By "Old Work Item" do you mean something that is resolved already or a work item that is still open and can have actions performed on it by a user?
As I understand it the situation is something like the below. Can you advise if this is correct?
A drop down is configured using propmt values defined on the property, and a value is selected "AAA" in this case.
If the Prompt values change and you then reopen the work item then the dropdown will no longer show the "AAA" (although this is still the current property value if you check the clipboard) value as it is no longer a specified prompt value.
If you submit this without selecting any value then the "AAA" value is lost, as it gets overwritten with the blank drop down selection.
I think what you are after is some kind of option for the dropdown to retain existing values even if they are not specified in the value list.