I lose reference field dropdown box when I change field type to Formula-Text any suggestions?

Sorry this posted before I had a chance to add my comments.  The issue I am having is with the Assigned Name New field from the Payments table (above).  In current state it is a Text field where we import the data but when Editing the record the user has the option to click on the field as a dropdown box to change the imported data from a list of User Names because of the relationship to the Profiles table (above).  When I change the field type from Text to Formula-Text I lose the dropdown box and of course the ability to change the User Name.  Any suggestions would be greatly appreciated!