You can easily add 1000 to all of the current users' IDs. Then you can reset the autonumber for the table to continue from there.
That can all be done in the database directly with the following two SQL statements.
Update pm_users SET UserID = UserID+1000;
ALTER TABLE pm_users AUTO_INCREMENT=1201;
(look up the largest UserID and set the auto increment to one higher than that, 1201 is just an example)