Implement subscribitions to an existing site
 

The issue is pretty straight forwards, but I can't figure out how to proceed.

I have a commercial site, and a support forums. People who purchase anything on commercial site gets right to register themselves into forums, and i manually approve registrations for them to be able to access threads.

When moderation gets approved users automatically moves from COPPA to Registered Members usersgroup.

Now, for some existing forums I want to implement a paid registrations

• Registered members will have access to this paid forums for one year from the moment I activate the paid registrations.
• New users will register for forums for free and get in COPPA usergroup, I will review and approve their registrations as I was doing, but as soon as they complete one year, they will have to pay to extend their registration

Can someone show me a way how to proceed on this?

Anyone got an idea on this?