I also found out it is possible to switch to BETA from Nightly via CLI
Possible but not supportable. The nature of nightlies is that they are very, very lightly tested. Things may be in such a state that moving to Beta just won't work from there. My crystal ball tells me that this is why moving from nightly to beta/release is not supported.
Personally, I'd rather see the occasional bug slip through to one release after target, than have the forums full of people who went to a nightly in order to fix a bug and then all hell broke loose on them.
If you think "but there is a clear warning", read the forums around the release of 12.0-Nightly. Several people who needed to be walked through boot environments because they did not, actually, comprehend that "Nightly" means "this will most likely break on you in new and interesting ways". People don't read warning prompts, as a rule.