I think Nimdock has got it bang on!
A new purchase-able service for perpetual and lifetime holders that is $20 a year for all new services you plan on offering now, and in the future. Perpetual have the opportunity to pay $30 a year for support or $50 a year for support + services.... both can have different renewal dates if needed, it's two different 'products' As for the Lifetime holders, they can just choose to add on the services license, which is $20 a year, or $50 if it has to match. Sorted.
I think that covers most of the issues, and a fair deal for everyone? I know that if I was a perpetual or lifetime holder that I'd be happy with these terms... makes a lot of sense: you are still contributing to paying for the service (the same yearly fee as a current standard), but your license terms remain in tact.