I'm not 100% sure what you want to do, but afaik anything is possible as long as you update. Anything going back in time is explicitly not supported and might break your system. So stable -> snapshot is fine, snapshot -> current is fine, but once you are on current, I believe you are stuck there, at least until one of the other branches overtakes current. I have switched successfully from current to release in the past, but with the current version being older than the release.

Anyway, maybe this will help you: http://www.openbsd.org/faq/faq5.html#BldBinary