Allowing specific members in a 'Paid Membership' section?

[Cloverfield]

So basically I had an 'Elite Section' where if you had over 500 posts you would be upgraded to a new usergroup and this usergroup had access to the elite section. I now have changed it to a 'Premium Section' where you can only access it by paying the yearly or lifetime fee. However, it's unfair to ask the members who were allowed access before to pay to enter the section and get a whole new usergroup so how can I allow specific members into the section whilst keeping the subscription service active?