NixOS is the first Linux distro that I'd dare to swap out a new DE with.
I've broken past installs (Arch, for sure) by trying to do this on other distros.
Obviously, make a full backup (unless you're on NixOS, in which case, everything just gets rebuilt deterministically from a definition file in either direction, and all previous instances are saved, permitting trivial rollbacks)
On Arch this stuff is easy. I use GDM as the login manager and launch pretty much whatever DE I like from there at login time. I use nix too, though rootlessly with nix-portable.
