Way to auto promote Additional->Primary group?

[GoodOmens]

Is there a way to automatically change a users primary group from a specified group he is a additional member of? Then have his primary group changed to a additional group?

I know this can be done on a user by user method but I was wondering if there was a way I can do it automatically via script etc?

Even better would be when the users request to join a public user group is approved his status gets changed on the spot.

To further complicate things I want to restrict the promotion to primary for only my main registered user group (To keep groups like Admin and Moderators from being changed). I did some searching and couldn't find anything.

The way I see it working with the current method is a promotion that will promote a user if his primary group is X and he is a additional member of Z change his primary from X->Z and add him as an additional user to X.

The reason I'm asking is because I use a mod that looks at the primary user group for permissions.

Thanks!