Probably, it should not be done because UserID is connected to many other tables.
If files were not changed and/or are not of great value - the simplest option - to enter in vault as Admin and to cancel checkout. After that with these files it is possible to do that you want.
If such option does not approach, it is better to execute experiments in test vault.
John, Please do not do this :-) There are many locations where the UserID is logged. Please open an service request (SR) with your VAR and have them work with out TS team to find a suitable resolution. Regards, Joy