Legality of Changing Open Source Licenses?
 

If one were to create an open source program and license it under the GPL, they own the copyright on that software. However, let's say that over time the project becomes very large and I have 100+ developers who have contributed patches to it. If I decide to change to the BSD license, is it legal? I would think that may be a bit decieving to the programmers whom I've accepted patches from. They thought that their work would be under the GPL and now the main author is changing to the BSD license. Is this fair, and is it legal? Would it even be legal if I got approval from all the developers who contributed patches?

If you created the program, and even though they made patches, you still maintained it.. then I would say you probably could do anything with your software.

If you get permission of all the developers who contributed then yes you can change the license. If one of your contributers does not like the new license you will have to remove their code before changing licenses. Anything they have contributed will be under the GPL and they own the copyright to it, you don't. If any of the developers give up copyright to the code then you don't need their permission to change the license.

However the new license will only apply to the code from the point at which you change the license. So all old versions will still be under the previous license.

Some projects fork when some people want to change license. But still any code that is still under the old license(ie the developer who wrote it did not give permission) has to be removed and probably rewritten.

It would be impossible to change the license if you can not contact any the developers as they could reappear and cause issues. It is best to choose a license at the start and stick to it.

Yep you can't re-licence something unless you hold the copyright or it is in the public domain. For example the guy's over at Boost.org have a blanket permission page to try and avoid such a messy situation.

Note there is a line in the GPL comments at the top of each source code file which says
That bit in bold has proved very controversial in the past and some GPL2 project do not include it!