When, if ever, does this need to be changed. I installed 18.09, but have upgraded to nixos-19.03. Would now be a good time to change the stateVersion from 18.09 to 19.03?
Same as When should I change system.stateVersion?
Basically: Read the release notes and don’t change it until the release note explicitly tell you to do so.
So, essentially, never?
You basically set it once when you’re installing a completely new system and practically never change it.