You can always restore a backup of an older version of iOS onto a newer version of iOS, just not vice versa.
If you really want to install an older pre-release version of iOS, then you can if you still have the IPSW. Apple doesn't stop signing pre-release versions like this so you should still be able to install an older version through iTunes.
As no answers are forthcoming, I'll report here my findings.
On 2015-09-16 , before iOS 9 GA, I removed the "Beta" profile from the iPad and rebooted. After reboot, software update says:
iOS 9.0.
Software is up to date
The version of the software is 9.0 (13A4325c)
So it looks good. There's an oddity, though. When I've rebooted (and hard-restart too to be on the safe side), every time I unlock the iPad I still get a notification saying:
There's an update available. Update the iOS 9 beta version.
I hope that when 9.0 final comes up, the update check will pick it and I'll be back into mainline versions. I'll update here when things change.
Edit 2015-09-16 19:39 CEST: Apple has just released v9.0 final and going to software update in Settings I get prompted to upgrade, so it looks like I'll be able to get back to mainstream. The notification after login stills says the same. Will report later when I have the chance to upgrade.
Edit 2015-09-18: After installing the 9.0 update, and the subsequent reboot, the device says its updated to 9.0 and I no longer get the prompt to update -- so all seems to be fine!
Edit 2015-09-25: Just a final update to confirm that after a few days running, all seems to be fine. I've just got the prompt to update to 9.0.1, so I seem to be fully in "mainstream" now.
Best Answer
Are you saying that your firmware is higher than iOS 9.1 and you want to downgrade back to 9.1? If so, you can't because Apple has stopped signing iOS 9.1 since December, I think. You can learn more from this post.