No. All you can do is backup your own data and forum, and reinstall.
In the future taking a snapshot before upgrading and deleting it after you are sure all went nicely and you want to stay at it, is really good policy.
If all you have is a forum, then backin up, spinning a new VPS, installing Disourse and restoring is the most easiest way. Or you stay in that and upgrade when next LTS is matured enough. That is an option too.