[35fbe7d3d5b9]

> If you release v1 and v2 under GPL and then switch to a proprietary license for v3, I'm assuming v2 can still be forked, since many people now have started using that code under the license they were offered at the time?

That is correct. Once v2 has been released under the terms of any permissive license – GPL, 2-BSD, MIT – and you get a copy, you are free to continue following the terms of that license. The holder of the copyright cannot retroactively change the old license.

[hnarn]

What about legal demands on the original authors? In this case, say an end user has a copy of v2 but only in binary form, can they then legally demand the (v2) source code from the company and claim breach of contract if they refuse? AFAIK it’s a central part of the GPL to be able to request source code, and presumably especially so in the hypothetical case where you have bought it from them?

[35fbe7d3d5b9]

This is definitely "ask a lawyer" territory but I'd expect those demands to be seen favorably by courts: the clear intent of the GPL is that someone who is allowed to have the v2 binary is also entitled to the v2 source.