Userid is used all over the place as a foreign key, changing it is a really bad idea. You will need to change access, administrator, adminlog, usergroup, announcement, announcementread, article, attachment, calendarmoderator, cpsession, customavatar, customprofilepic, post...
I cant be bothered listing thhe rest, but basically it is most of the tables.
Exactly.The same also applies to forumids.I did try to alter them in the tables once and the ids placing got screwed up!Not advisable to!