Copyright assignment is an interesting topic.

With the copyright assigned the owner of said copyright can release the code under any license of their choosing.

Some dual license open source projects do this, the organization requires the copyright assignment and hence they can release the code under an open source license and their own closed license. License changes here are perfectly OK (legally at least).

Interesting cases are those without a copyright grant. Now you allow the group to only produce copies and releases in accordance to that specific license. License changes now are questionable. Can you relicense Apache 2.0 as AGPL? BSD as Apache 2.0? (Without owning the copyright, my guess in both cases is no.)