When he had a usergroup change based on postcount, we did it with some homemade approval queue. We'd check their posts behavior and see if they were ok to stay or not.

It was for 6.5.5, but was basicly a list with everyone in the old group that fit the requirements (in our case, plus 100 posts), with links to view their post and profile.

[Linked Image from siemons.org]