I've had two experiences with changing the datasource, or list of selectable value, in a drop list.
1. The original value on old forms still displays and actually stays as an option in the drop list until it is changed - at which point the user can't get it back.
2. The original value does not display, because it is not a value in the current drop list - but the value actually still is in the field.
Both results seems to be dependant on how you retrieve your values (e.g. through SQL data connection).
I still think there is a couple of lateral solutions which you could try (not sure if it will work)
1 have a read only text box above the drop list which displays the current value - old values will still display
2 if you are changing the available options - this usually means the old options are no longer relevant, and should be removed or replace with one of the new ones. Try a Open rule on the form which updates the field's old value to a matching new one, if you are using dataconnection for Main datasource - you could put the rules in code on the query event.