In that case, there are two issues that need to be covered by FB; first, they need receive an agreement from all collaborators they agree with the license chang

In that case, there are two issues that need to be covered by FB; first, they need receive an agreement from all collaborators they agree with the license change—most likely, the SLA takes care of that; second, forks of the code up until the license change point still hold the original license—any fork that was created before the license change remains with the original license, and so does every use of code that consists only of code before the license change. This is what I meant by “retroactively”.