Let's say that Alice has started a project under a permissive license, e. g. MIT. Then Bob forked it and added some features. Now the project has two copyright holders, Alice and Bob. Then Bob decided to continue the development under a copyleft license, e. g. GPLv3 and fixed some bugs. Then Bob decided to move the project back to a permissive license, MIT.
The question: could Bob do this without consent of other copyright holders for the project, namely Alice? Intuitively he should have such right. But Alice's and Bob's work might not be dissectable within the project. Are there terms in copyleft licenses that control such situations?