> If they use their rights from the GPL, they must retain the GPL option for others (copyleft principle); if they use their rights from the CC-BY-NC-ND-4.0 lice

> If they use their rights from the GPL, they must retain the GPL option for others (copyleft principle); if they use their rights from the CC-BY-NC-ND-4.0 licence, they cannot make derivative works so won't be allowed to continue developing the project! If they own the copyright to all of the code that was published, then they can use that right to relicense the code however they like without violating either of the licenses. That would, however, presume that they either did not accept contributions from anyone else prior to the change, had contributions assign them copyright, or removed code by those contributors. And, of course, changing the license on new code doesn’t revoke the rights granted to people by the previous licenses if they had the code already.