Well you can try altering the table user i think and try changing the primary key of the user to 1
Oooh, such a really bad idea!
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.