@dfarmer I was able to transition from flakes to npins without a reinstall, but I did so cautiously. It's all just config files, so I made a copy and hacked that until it built again.
If your system is hosed, you'll probably want to revert back to a previous version of your config that builds properly. Then you can make changes from there to remove flakes in stages.
@dfarmer Ultimately, very little actually changes between flakes and npins. Both provide package pins, so the rest of the config just changes what source it's importing (flake.nix or npins/default.nix), and deleting a few flakes-specific settings.
If you're having specific problems removing flakes, I'd be happy to try to help. Just keep in mind you have to replace flakes with something, whether it be old-fashioned channels, npins, etc. Your installation needs to know where to get nixpkgs from.