I've been reading a bit about software licenses, and I'm having trouble getting my head around the idea of what happens when one wants to change the license. I've read a couple of questions here on the topic and it clarifies things some, but there are still a couple of areas that are unclear to me.
One example: Suppose I have a project hosted on github with no license specified. If someone contributes a patch to my code base and I merge it in, does that patch still belong to them? If so, would I need their permission to change the license on my project?
Another example: Suppose I have my software under some permissive open source license. I understand that I am allowed to change the license on my software at will (barring conflicts with contributors), but does that effect existing copies of the software out there? If I decide to close source my project, do existing copies of the source with packaged OS license remain open source and distributable according to the previous license?
I like the idea of working together with others on projects in a flexible manner, and unless a project actually starts to pick up it seems like unnecessary overhead to go about deciding on an appropriate license. It would be nice to be more aware of the potential conflicts that could crop up, though.