Demotion of users using standard Promotions?

[PGP]

I need to do a simple thing in general =)

But can't understand how can it be done with standard promotion system of vB.
Thinking that a need a mod for this, but maybe i'm wrong.
If anybody can help or give an adviсe - i'll be very grateful.

So i need to do only this demotion:

If user joined the board already for X days or more, AND number of his posts less then Y - then he moved (not pruned) to another group.

Returning user back into normal group can be done with vB promotion system =)
But how about demotion... I'm stuck here... =)