The rule is simple: If you want to relicense a project, then you need approval from all the individual copyright holders. It does not matter if you are relicensing between permissive OSS licenses such as, for example, from Apache 2.0 to MIT. They are different licenses, so you need permission for the change.
That's why OSS is commonly called a "Community". The software's copyright belongs to the hands of all the community of developers who have written and contributed code.
What a CLA usually does is grant perpetual permission to do anything with the code, including to relicense without asking. Practically speaking, CLAs grant full control to a single hand. Thus, OSS projects with such CLAs are not part of any so-called "Open Source Community" in any meaningful way.
EDIT: Changed "CLAs grant full ownership" -> "CLAs grant full control", to avoid misunderstandings.
