Hi Eric. You asked if it would make sense to start over with your default environment and I would say yes, it would be good to clear out the old format metadata. Let’s start by doing that:
flox push # to make sure you have a backup
flox export > /tmp/default.tar
flox destroy
flox import < /tmp/default.tar
flox edit # remove mention of prerelease
flox push —force # to remove old history upstream
That should do it - if any of those steps don’t work please can you repeat with —debug and send the output?
On the topic of prerelease, I suggest you switch to using the flox-managed flox/prerelease environment going forward. Not only does it provide you with the latest flox but will soon also provide the necessary magic to enable flox command line completion. Good stuff. 
(More background info on our plans for flox-managed environments is available here.)