Don't worry Robert even if the user Ids are changed for the users, UCCX still keeps the record for old users as well. For example, the user id was robmaule at one point for an user called Robert Maule and now you want it to be rmaule so that it can match what is there on the AD. In this case, when you will go to CUIC both the users will be showing up and you can pull the records for them individually since what UCCX does is, marks the user as Inactive and do not delete it completely from the DB so that if someone wants to pull the old records can still do that.
Hence feel free to make the changes, records will remain there. If you want to test it, you can change the userid for a locally created user and test it that way as well.
Regards
Deepak