As long as you don’t brick your phone you can always go back to FPOS, but as @AvidAlbatross noted, make sure, if you are locking the bootloader on a custom ROM, that that’s actually supported.
That’s generally what happens, since it’s a bug in the bootloader so Fairphone’s responsibility, but none of us work there, we can’t guarantee it, the policy might change any time.
Just a small addendum, if you decide to go that route and updated your phone to the latest FPOS already, you’ll need to use the beta factory images, not the ones mentioned in the instructions. More info here.