Updating the creator

Hi! We just migrated from a couple instance into one instance and put everyone on single sign on. In the process, we had to update the user name which changed the creator to a user-(5 digit #) I wanted to use the built in script to update the user but where it says creator = username if I put in user-12345 I get an error. Is there a way that I can select that user id to change it to their actual name?