You can, sort of, sometimes. Copyleft is still based on copyright. So in theory you can do a new license as long as all the copyright holders agree to the change. Take open source/free/copyleft out of it:
You create a proprietary piece of software. You license it to Google and negotiate terms. You then negotiate different terms with Microsoft. Nothing so far prevents you from doing this. You can't yank the license from Google unless your contract allows that, but maybe it does. You can in theory then go and release it under a different license to the public. If that license is perpetual and non-revokable then presumably I can use it after you decide to stop offering that license. But if the license is non-transferrable then I can't pass on your software to someone else either by giving them a flash drive with it, or by releasing it under a different license.
Several open source projects have been re-licensed. The main thing that really is the obstacle is that in a popular open source or copyleft project you have many contributors each of which holds the copyright to their patches. So now you have a mess of trying to relicense only some parts of your codebase and replace others for the people resisting the change or those you can't reach. It's a messy process. For example, check out how the Open Street Maps data got relicensed and what that took.
alt Hacker News
You can, sort of, sometimes. Copyleft is still based on copyright. So in theory you can do a new license as long as all the copyright holders agree to the change. Take open source/free/copyleft out of it:
You create a proprietary piece of software. You license it to Google and negotiate terms. You then negotiate different terms with Microsoft. Nothing so far prevents you from doing this. You can't yank the license from Google unless your contract allows that, but maybe it does. You can in theory then go and release it under a different license to the public. If that license is perpetual and non-revokable then presumably I can use it after you decide to stop offering that license. But if the license is non-transferrable then I can't pass on your software to someone else either by giving them a flash drive with it, or by releasing it under a different license.
Several open source projects have been re-licensed. The main thing that really is the obstacle is that in a popular open source or copyleft project you have many contributors each of which holds the copyright to their patches. So now you have a mess of trying to relicense only some parts of your codebase and replace others for the people resisting the change or those you can't reach. It's a messy process. For example, check out how the Open Street Maps data got relicensed and what that took.