usergroups auto change

[ChipW]

I'm not sure if the title did my query justice, but here it goes.

I'm looking for a mod that will put new users into a specific usergroup and after a set number of post, automatically move them to another set usergroup.

The reason for this is basically anti-spam. I can have the initial usergroup postings be moderated and catch spam or potential spammers before they can actually post anything. Then, be "promoted" to an un-moderated usergroup automatically.

My board is geared toward a very specific audience and it would be easy to catch someone trying to lie their way to the next un-moderated usergroup.

If anyone knows of a mod that will do this or is possibly willing to make one for free , please let me know.