Seriously - **DON'T!** (did I stress that enough?) Not unless you've tested it well on a non-production machine. It's been known to break themes and plugins. My Advertising Manager plugin is still broken as of this writing, and there's no automatic downgrading.

If you were hasty like me and ended up getting in trouble with the blasted 2.8 version, check this link on some steps to perform the downgrade.

http://christastjean.co.uk/