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.
I found this from the Apple support site:
"Download the final version of iOS
If you have an iOS beta version and want to download the final version, follow these steps:
Connect your device to iTunes.
While your device is connected, force it to restart. Press and hold the Sleep/Wake and Home buttons at the same time. Don't release when you see the Apple logo. Keep holding until you see the Recovery Mode screen.
When asked, choose Update to download and install the latest nonbeta version of iOS.
An update install won't affect your content or settings."
I would recommend backing your iOS device up to iTunes before attempting this.
Best Answer
You don't need to upgrade. OS X 10.11 GM and final release are the same according to Apple and once 10.11.1 is released both the GM and final release will update to it.