Your field validation can be different from the search validation. In fact, most of the time they should be different because you want to include expired (disabled) values in your searches but exclude them from being input on new requests...
correct, the search validaiton can be different. But the problem is: if you have field type as "Text-20" then search validation field is disabled. It's enabled only for the auto-complete field validations.
So we want to change the original field validation to Dropdwon form Text, in order to enable the seach validation field.
Well in your case the only potential issue I can think of is if you have requests that have data in that field that isn't exactly matching the values of your new validation you would want to update them. Otherwise they will not show in searches and they will ask the user to update them at the next save.