If they accepted changes from someone else, that someone else has copyright on those changes and the license can't be changed without them agreeing to it (or possibly their changes being removed from the codebase; I am not an IP lawyer and you should consult one if you want to be sure on this score).
If they accepted changes from someone else, that someone else has copyright on those changes and the license can't be changed without them agreeing to it (or possibly their changes being removed from the codebase; I am not an IP lawyer and you should consult one if you want to be sure on this score).