In the NixOS upgrade instructions, it warns that:
Warning: It is generally safe to switch back and forth between channels. The only exception is that a newer NixOS may also have a newer Nix version, which may involve an upgrade of Nix’s database schema. This cannot be undone easily, so in that case you will not be able to go back to your original channel.
Where exactly do you incur this potential problem and pass the point at which it is difficult to revert to the previous channel? Just by changing the system channel using
sudo nix-channel --add <channel> nixos? Or by doing that and then running
nixos-rebuild --upgrade dry-build/dry-activate? Or is it not until
nixos-rebuild --upgrade boot/switch?