Even though GPL is the most obvious case where you'd want this, it's not the only one. AFAIK there is no software license which prohibits the copyright holder (ie original author) from relicensing their software, either to individuals or openly, under as many other licenses as they see fit.
It does get complex when a single Open Source project contains code from multiple different contributors, each holding their own copyright. I believe that's one reason why plenty of OSS projects (GNU & Android both spring to mind although with opposite motives) require contributors to assign copyright as part of contributing.
Alternatively, it's also a problem if you (say) link a bunch of GPL-licensed libraries to your own OSS source code. AFAIK in that case you can't really relicense & distribute your project in its entirety under a different license, due to GPL virality.
I have NFI if any of these gotchas apply in this case, though.
It does get complex when a single Open Source project contains code from multiple different contributors, each holding their own copyright. I believe that's one reason why plenty of OSS projects (GNU & Android both spring to mind although with opposite motives) require contributors to assign copyright as part of contributing.
Alternatively, it's also a problem if you (say) link a bunch of GPL-licensed libraries to your own OSS source code. AFAIK in that case you can't really relicense & distribute your project in its entirety under a different license, due to GPL virality.
I have NFI if any of these gotchas apply in this case, though.