If your project insists on copyright assignment (as the FSF does) then there's a single copyright owner and the project can make any licensing change it likes.

If your project insists on copyright assignment (as the FSF does) then there's a single copyright owner and the project can make any licensing change it likes. Otherwise (assuming you don't manage to get explicit agreement from all previous contributors for their past changes) the only licensing term changes you can make are those which the previous license already permitted. For instance, many projects use a "GPL v2 or any later option" wording, which would allow them to later move to GPLv3 even though they had previously accepted contributions made when v3 didn't even exist. In this case it looks like jQuery was previously dual-license MIT-and-GPL, so anybody who contributed to it was saying "you can use these changes under either MIT or GPL". That means it's entirely OK for the project to switch to "just MIT", because "MIT" was an option all those contributors were permitting already.