There is more than one table that calls the usergroup ID.. :erm:
FYI.. there are 85 matches for "usergroup" in the vbulletin schema, which you can find in your install folder..
You will break the site if you changed one usergroup ID as you said..
Really? Fortunately I was testing it on my test-site, so there's nothing to be worry about.
Yeah, I figured out changing usergroup table didn't work, then I reverted changed back!
And now my test is done via another method