[iurisilvio]

AFAIK, changes to this GPL codebase must be GPL.

Licenses are always complicated, but when I contribute to a project GPL licensed, I expect any changes to be GPL too.

I'm not a lawyer, but studied licenses to understand which projects I can use.

[akerl_]

The author of the code can change the license for future releases. So if I write a GPL'd project and distribute it, I can decide at any point that I want the project to be BSD or closed source or anything else instead. Anybody with a copy of the old code can continue adhering to the GPL for those old versions, but new code follows the new license.

Any obligations I have under the old license still stand (for example, if I shipped precompiled binaries of my old GPL'd version, I still need to make the source available at that version for people who have the binary.

It gets more complex if, as the other commenter here mentioned, there are multiple contributors. If I merged patches into my project from 10 people, I need their permission to change the license of their code. So either I need to ask them, or I need to get them to assign copyright for the work to me so I can decide for them (this is part of the reason why lots of larger orgs / projects ask contributors to sign a contributor agreement that assigns copyright to the org).

For an example of how this looks in practice: OSX still ships an ancient version of bash, which was released under the GPLv2, because newer versions are released under GPLv3 and they're concerned about including GPLv3'd things due to the additional terms in the license. They're free to continue using the old version of the code under GPLv2, but can't apply the old license to the new code.

[asteadman]

This would normally be the case, but for small projects with a small number of contributors, they are always free to dual-license their own code, even "retro-actively". Anything that was already released would still be available under GPL, but new edits could in theory be licensed under whatever terms they want, independent of how "derivative" it is. For large open source projects without a copyright assignment clause, this is essentially impossible, but for small projects it's relatively easy, assuming you can track down all the contributors. IANAL.