I put 998 on last night and had a quick faf about, all good, but I have been using 99e on a track I'm working on, it uses record trigs and due to the quirk of 99e record trigs it only sounds right on99e, so I reflashed from 998 back to 99e, rebooted and got a error message right after the boot screen, the OT was locked up. Panic set in, I rebooted a few times in the hope it would clear - nothing, so then I tried to access the edit menu, no joy there either, started thinking it was going to have to go back Anyway, I removed the card and tried an empty reset and luckily it cleared the error, so I'm thinking that the file structure has changed, so if you plan on going back to an older OS it is probably wise to do an empty reset before reflashing to an older version.
Possible Solution: I think the problem might be that the OT autosaves and autoloads whatever you were last working on and as soon as you boot with a new OS changes are being written to the project file that are no longer backwards compatable. One solution in this case would be to downgrade the OS and before turning the OT back on replace the project files on the card with a backup made before the OS was upgraded. I doubt the OT was nearly bricked, just incompatable data loading from the card is my guess.. I think the problem might be that the OT autosaves and autoloads whatever you were last working on and as soon as you boot with a new OS changes are being written to the project file that are no longer backwards compatable. One solution in this case would be to downgrade the OS and before turning the OT back on replace the project files on the card with a backup made before the OS was upgraded. I doubt the OT was nearly bricked, just incompatable data loading from the card is my guess..