Copy the client. Assign a number that indicates it's the original; we append "AOF" for "as originally filed." Amend the already-REPped file with the unchanged number.

I believe that the current system won't charge a second REP fee if the last name, SSN, and states are unchanged, even if the client number changes (or is new). But I don't believe in it so strongly that I personally re-REP every copied file. I tend to use the all-9s as SSN method for what-ifs involving multiple states, and having an un-REPped copy of the file for the original when something needs to be amended.