I think the backpack model is the way to go. Everybody can get free access, but there are some arbitrary limits. Pay $5/mo or year and those limits go away. Also, there's no reason to disturb existing customers. Never never, take anything away that you've given people for free. If you feel your new feature isn't the thing to limit, be sure to roll your existing users into a special $0/month premium account. just ask new people to pay.