A huge part of that is how to structure an open source project so that it can't be taken from the community. Basically a combo of GPL and taking community contributions without having them assign copyright to you is the most full proof way to keep something open source.
To answer your question, if they own everything they can relicense it any way they see fit. Only by having your code mixed with others' that you don't own can you practically protect it from forced relicensing.
