> Last year I released something under the GPL, and when a few potential users complained I wimped out and switched licenses. I think that I made a mistake doing that.
If it's your own code, you can change licenses back (say, for the next version) if you want to.
If their contributions were licensed to him suitable for inclusion in a project released under a permissive (and GPL compatible, but most are) license, then he could relicense his code and thereby effectively relicense the project. People could, in principle, isolate the BSD (or whatnot) portions and use them in other projects, but that's probably only relevant if it's a significant portion of the code.
If it's your own code, you can change licenses back (say, for the next version) if you want to.