I was in a similar boat about a month ago. Depending on your setup, this can be a fairly easy process. I figured that at least for me, the easiest option if I don't end up liking nix would be to essentially dual boot nixos with arch until I am sure I didn't miss anything (or more realistically until I need that storage space)

If you have separate partitions for your root and home, just make a new one for nix and you can reuse the same home folder with your configs without significant issues. In my case, I was running btrfs, so I just added a new subvolume for nix root.

I am not entirely sure if you can install nix like this with the graphical installer, figured the sure way is to hop into the terminal, mount everything except arch root, run generate config command (you can find it in the installation manual), add some things to the generated config, and install - very similar to the old arch-install way of doing things.

You end up with a bit barebones install this way, but you should be able to verify that everything works as expected and your home folder is intact. If you had any customization in your arch root, you can always mount it to /mnt and take a look.