The documentation says that changing the domain on your AIO instance isn't supported, but it's actually quite easy.
You just have to edit a json file and replace a few strings in the database.
I wrote a PR to update the docs, but Nextcloud team thinks it's a better idea to just manually go through every app in your admin settings and just manually change every occurrance of your old domain by hand; but this is tedious, more error-prone, and liable to miss references.
Instead, I suggest just doing a few replace() calls in your database. It's worked for me without any problems. Steps below: