I am creating a game in C++. Now I am not sure on how I will distribute it yet - though I am pretty sure that I won't be asking money for it. And I am looking into a licensing and I wondered if there is a license that is suited for the undecided like me.

My current releases (which are really really early versions of the game, with far from full functionality) are executable only. However, I am actually thinking that I might release the source on an open source license. For now, I am the only contributor, so that would be no problem as I only need my own permission to move to a less restrictive license. However, when I allow other people to contribute, I would need all their permissions to do so (right?). So I was wondering if there is a license that let's me distribute the game executable only for now, but will let me switch to a less restrictive license if I want. Basically I need a license in which contributors give permission to switch to a less restrictive license up front.

Does anybody know of license (or other construction) that would allow me to do so?