When a user purchases a User Upgrade they can be promoted to a user group.
I would therefore suggest allowing any user to register (without a charge), but to access any of the forums and features they would first need to purchase the upgrade.
So a normal registered user may not be able to do anything on the forum besides purchasing the upgrade, but once the upgrade is purchased, that will put them in a user group that will allow them to access forums and other features.
That would happen automatically when the payment is completed. Your forum will receive IPN messages from PayPal, so should the payment be reversed (chargeback) or you refund them, or they stop paying their subscription, PayPal will notify you and then automatically demote the user to a normal member. At which point they'd lose any access to member forums, and other features, until they paid again.