Internal IDs are used as identifiers and they link together all your posts/threads/pm and loads more. You can just delete userid 1 and then change your userid, you would need to change it across all the tables, rebuilding any caches e.t.c.
UserID don't mean anything to the end user... I don't see why people have a fixation with them. Its only a number.
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only time you are going to see it is on the member profile in the URL or the User Manager in the AdminCP. (I think).