Why not just use the built in subscription feature..
Yes this is something I'd like to do but I don't want to interfere with payments (subscriptions) that are already running. I just assumed that I'd have to get everyone to start a new subscription and cancel their old PayPal subs?
Not knowing how you setup your existing subscriptions.
There should not be a impact with paypal.
It would be a different transaction ID.
So as existing people start to roll off advise them to use the new built in subscription to renew.
With that said I think you still need to create a net new custom group duplicating permissions from the one you use now.
So UBB can monitor and add/delete members on it's own.