Hi Rob.
Welcome to the forums.
Follow the upgrade path outlined in the firmware release notes. I think it outlines 4.0MR3 patch 10 (or higher) is the official supported firmware for upgrading to v5.0GA (do not have the release notes handy to confirm).
If the 60C is on 4.0MR1 or 4.0MR2 you will likely need to upgrade it to 4.0MR3. Once on 4.0MR3 the upgrade path is something like 4.0 MR3->4.0MR3 Patch 1->4.0MR3 Patch 5->4.0MR3 Patch 10->V5.0GA patch 1.
Things to watch out for: Disk logging is not compatible (v5.0) -- you may need to re-format your log disk after the upgrade. You will lose some settings (release notes will point this out).
Yes it is possible to roll back (check the release notes).
Learning from personally experience -- follow the upgrade path as closely as possible -- I upgraded my wifi 40C from 4.0.MR3_patch 5 to 5.0 GA patch 1 and ended up having to reformat the boot device and tftp a new firmware image onto it.
The actual firmware upgrade process only takes a few minutes (assuming you only need to perform one upgrade).
Back up your config before every upgrade -- confirm everything is functioning properly after each upgrade. Perform a " diagnose debug config-error-log read" at the CLI to see what went messed up during the upgrade.
Enjoy Life.
NSE4/FMG-VM64/FortiAnalyzer-VM/6.0 (FWF30E/FW92D/FGT200D/FGT101E/FGT81E)/ FAP220B/221C