So it is possible to directly upgrade from 8.4 beta 3 to 9.0 beta 1. I was searching high and low for this answer. Maybe can do a full wipe?
You can using iTunes... No promises on how well it will work though. It has worked fine for me. My phone is registered but I just thought I would give it a try before restoring.... As it's kind of a pain in the butt to reregister everything on Apple Pay :/ but it's worth a try... Just back up your device first and if it doesn't work do a dmu mode restore.