This was an entirely seamless upgrade for me. nixos-rebuild switch --upgrade + 5 minutes waiting == new shiny. I haven’t had this easy of a time upgrading since I left BSD-land.
nixos-rebuild switch --upgrade
I’m doing this at the moment, after having read your comment. But it’s now 1 hour and it still is not finished. It compiles quite a lot of stuff, that it didn’t in 20.03 - I could see Thunderbird and Libreoffice, just by looking from time to time to the console. I hope it will finish soon. Honestly I hope updates will be much quicker.
Another difference that I have seen, is that it doesn’t accept to run plasma5 (KDE) and gnome3 at the same time - reported a conflict, so I removed plasma5. It’s strange, because in 20.03 they worked.
EDIT: Update, finally finished to upgrade from 20.03 to 20.09. Took ca. 3 hours. I don’t know why but a lot of programs that normally were downloaded already compiled, this time had to recompile.
One thing that really annoys me with NixOS releases is that .09 almost never comes out in September, nor .03 in March.
From what I can tell those being in charge are aware about that annoyance. See https://discourse.nixos.org/t/what-should-stable-nixos-prioritize/9646 for a discussion about doing better for stable releases.