I hate to lose lifetime licenses, but in this case I would agree. Although, lifetime access to downloads seems more common practice, access to support should be probably paid as Craig said at 30-40% discount.
Tom, not to offend early adopters (like me:)), you may freeze the current price for current clients, then you may raise the price once in a while for new clients. Your product now is not the same as it was.
You may also think of converting some addons to commercial plugins that work with any themes. Despite, there are similar ones, such addons as sections, menu+, page header can be competitive. I heard you planned to release more advanced child themes, great idea! Maybe restrict access to the forums (as Hybridtheme), maybe divide the support into basic (usage and bug) and advanced (customization) …
I don’t know, somewhere somehow your price should be higher). Good luck with changing the model, tough decision ..