I've got a device that's on the 8.4 beta, and I'm wondering if it's safe to just skip the rest of 8.4 testing and go straight to 9. Thoughts? I couldn't find anything in the installation guide that discussed it.
Safe to go from 8.4 beta straight to 9 beta?
If you don't care about iOS 8 anymore, then you should go ahead and start developing for iOS 9 now.
Best way would be a full restore to be honest. Always restore when going to a Major Beta as upgrade paths might provide some unexpected results.