Hmm, something I’ve never considered… What happens if an open source project changes its license and becomes proprietary? I imagine all forks would need to purge the adopted code.
No, not at all. Once you release software under an open-source license like the GPL that is irrevocable. That specific released version will always be available under the GPL. Forks can continue.
Now you own all the code you produce, so you can release it under the GPL, or commercial license, or whatever you want. You can change code previously under GPL to proprietary for new releases too, and stop distributing the old GPL code, although you can’t stop someone else from distributing that GPL code. Or you can release code under the GPL and also license it commercially under a less restrictive terms, so a company can integrate the software into their commercial product for example.
But if you accept code from third-parties, you would need to get their permission to license their code commercially, which would be very difficult to do. Or otherwise, remove all code from contributors.
Many OSS projects have contributor license agreements, and you need to carefully read them before deciding to contribute, as many of them flat-out sign over your rights.
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