AFAIK, if you're still running mbox, then Neomail should continue to work. Nick has mentioned in the past that it would be deprecated in the future, so whether that means an enforced upgrade to maildir (and effectively the removal of Neomail) or not, I don't know. I'd be surprised if they did enforce an upgrade, but they may decide to drop cppop and that will be that.
You can, in fact, downgrade a clean system from maildir back down to mbox if you really (really) want to - but that would seem a huge step backwards.
You could try contacting them directly and ask if they have any hard plans for a forced move to maildir in the future.