[sql]UPDATE user
SET userid = '1'
WHERE userid = '4';[/sql]

[sql]UPDATE user
SET userid = '4'
WHERE userid = '1' AND username= 'username';[/sql]

The above two queries should work, I didn't test them. And I also don't know how this will affect your board. So be warned, and I would test it on a test board before you do it.

And Change 'username' with the username of the user you want to switch.