What requirements you have to satisfy to change your software license?

Question

Let's say you have published your work under GPL, but after some time you realized that it would be better to publish it under other, GPL incompatible license? What options do you have? Are you (or your team), as an author, allowed to simply change the license? Or do you need a permission from each contributor, each bug finder?

I know that some teams were changing licenses of their software, but I didn't find much info about how to go through this process. I don't want to close some paths before my software or definitely stuck inside one license.

Answer 1

You need permission from every copyright holder. Note that copyright holders need not be contributors, they could have acquired the copyrights other ways.

To protect yourself, generally one gets this permission in written form, signed by the copyright holder -- this is typically called a copyright waiver. This usually includes an explicit acknowledgement that the copyright holder give up their rights to their copyright on the software.

Once you have waivers on file, you can change the license however you see fit.

Answer 2

If you have merged in bug fixes then getting permission from every copyright holder can be difficult or impossible.

An alternative option can be to branch your project and remove all code that you (or your team) do not have copyright permissions. You can then relicense this untainted code under a new license of your choice. Note that once code has been merged and worked on further since it can be difficult to prove which code belongs to who so you should contact a lawyer.

Some projects also require all contributors to give up copyright to the project owners when submitting patches, etc. This makes the process of changing license much easier.

Answer 3

Keep in mind also that you have already released your code under the GPL, and each recipient of that code is entitled to send the code to everyone they know, still under the GPL. You cannot take back the rights that you've already given them. Since you (and the other copyright holders) are the original owners of the code you are allowed to modify the code and redistribute the new version under a more restrictive license.