Technically true, you can only offer a license on code you've written. But effectively you can still take a MIT/BSD project, and fork it and release as another license, legally.
The original code will still be under the permissive license. But you can take it, and fork it, and continue to develop the fork, and all your new additions can be licensed under the more restrictive one (including GPL).
To be clear about what's going on, you could put a notice saying that anything taken from Source Project X was MIT, all further changes were (GPL). Perfectly legal. But really, how would people know which parts are which except diff'ing it, they might as well just go back to the original -- or they can always diff if if they want regardless of license of course. MIT/BSD don't require attribution or anything, they just say do whatever you want with the code.
So forking and just saying "the license is [eg] GPL" is pretty much the same thing. It's legal. You can't stop people from continuing to use the original code without complying with the GPL, but your fork is GPL.
IANAL. Maybe if it ever got litigated it'd end up different. It might not get litigated, for a while.
The original code will still be under the permissive license. But you can take it, and fork it, and continue to develop the fork, and all your new additions can be licensed under the more restrictive one (including GPL).
To be clear about what's going on, you could put a notice saying that anything taken from Source Project X was MIT, all further changes were (GPL). Perfectly legal. But really, how would people know which parts are which except diff'ing it, they might as well just go back to the original -- or they can always diff if if they want regardless of license of course. MIT/BSD don't require attribution or anything, they just say do whatever you want with the code.
So forking and just saying "the license is [eg] GPL" is pretty much the same thing. It's legal. You can't stop people from continuing to use the original code without complying with the GPL, but your fork is GPL.
IANAL. Maybe if it ever got litigated it'd end up different. It might not get litigated, for a while.