A lot of things you could do. I think, if you have phpMyAdmin or something, you can delete all users except for 1. As it's auto-increment, the next member that joins should be 2. Or, rebuild the entire table from scratch (though, the UID's are linked elsewhere, such in posts... so you'd have to modify them too).
Then, just change the post content ownership when the new members join to match who posted what.
Otherwise, start from scratch.
It's a little OCD to be thinking that missing a UID is important unless you just want the top 25-100 to have a claim to being that user ID down the line. If you think UID is important, wait until you get into having 10+nodes and then finding out you need the 11th node, but in the 1st category so it goes node id 1, 2, 11 (you can remove node ID in the URL portion), but, the IDs are irrelevant IMO. You're going to have UID gaps, I believe when it comes to profiles when it comes to banned/unconfirmed/unregistered or something because I have X users, but UIDs are 35+ more than actual members and I never deleted any.
The easiest, and probably recommended way, would do a fresh install as you have so few members, and likely few posts too. Save all posts (if necessary) and then repost them under in admin in a private node, reassign them to the new accounts with an add on after they rejoin, then move them to the new forums created.