Unfortunately I can't see any procedure how to re-image FTD HA pair to the previous major release.
1) Unregister (delete) FTD HA pair from FMC. This step is needed since after the FTD reimage its UUID changes and cannot be registered again, but has to be added from the scratch.
2) Go to FXOS WebUI and delete/recreate FTD VM under Logical Devices for the 1st unit with another image
3) Go to FXOS WebUI and delete/recreate FTD VM under Logical Devices for the 2nd unit with another image
4) Reconfigure the primary unit manually in FMC
5) Join FTD units into HA pair in FMC
Assume FMC is not downgraded and runs the latest release, step 4 in my opinion can't be recovered from backups?
maybe somebody has a real life experience.
