Would it be possible to simply fork an MIT-style license and relicense the new fork as AGPL? The pre-fork version can continue with its old license, but the new fork is AGPL.
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.
You can't just change the license of code contributed by someone else without approval. But you can relicense your own contributions, which - unless that code can be trivially ripped out - would basically have the same effect as placing the whole thing under the AGPL.
You can't really just 'slap a new license on it' AFAIUI. What you can do is fork and license any new derivative work under a new license (which is one-way compatible with the old one). Effectively that means that the fork can only be used under the new license.
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.