Just 1 advice: Don't even try this, it is far more complicated then just changing the value in a table.
Besides there is no reason at all. The userid is just an internal number without a meaning.
without a meaning? i don't think so!
i have a forum, 1 month ago, hacker created user with user ID = 1111111.
And now, sometime i must update counter (Update User Titles and Ranks, Update Post Counts,...), process --> long time!!