[Laremere]

IANAL. The lack of forking isn't the issue here. When someone writes code they (or their company) owns the copyright to it. They then contribute the code to the project using the terms of the project's license. Eg, for GPL, it allows others to modify, build, and run the code. However those modifications must be released to the public under the same license. (skipping over some minor technical details)

Unless you get everyone who has contributed code to also release their code under the new license, the old license is the only one which all of the code has.

It is possible to start contributing code to a project under a new license (effectively re-licensing the project in the eyes of the community), provided that the new license does not violate the old one. Specifically the Apache license REQUIRES that the code be distributed with a copy of the Apache license. Just removing or changing that license without the copyright holder's permission is in violation of that copyright.

A lot of projects avoid potential future issues by having a contributors agreement in addition to the project's distribution license. Essentially, you give an extremely permissive (possibly up full ownership) of the code you write to the project. That is, some legal entity such as a person (the head maintainer) or a foundation. This legal entity then distributes the project to the community using the license of their choice.