Reseed next Drupal User ID (UID) Is it possible to modify the next User ID when creating new users in a Drupal 7 database?

As an example, 

I have a user at UID 100, I then create and immediately two user accounts (101, 102). When I next add a user, I want the UID to revert to 101 rather than default behaviour of 103.
In Drupal 7 the uid column in the users table isn't an autonumber, so there's nothing to reset/reseed.

When you add a user in D7 the user_save() function does this:

// Allow 'uid' to be set by the caller. There is no danger of writing an
// existing user as drupal_write_record will do an INSERT.
if (empty($account-&gt;uid)) {
  $account-&gt;uid = db_next_id(db_query('SELECT MAX(uid) FROM {users}')-&gt;fetchField());
}


So the next user ID is simply the next available free ID (current max + 1) from the users table.

If you have a record with a uid of 100, then you add 2 new users (101 and 102), the next available user ID will naturally/logically be 103; which will be returned by the above code in user_save().

If you need that to be 101 again for whatever reason, you need to delete the user you've just created that occupies the 101 ID, and set the uid manually on the new user record that you want to replace it, before you save it (as suggested by the above comments in user_save()).