The copyright owners of a GPL software can do whatever they want with future versions, even going proprietary. The problem is that all the owners must agree on that. That's why some GPL software only accepts contributions by people that give copyright to a single maintainer entity. An example is FSF's copyright transfer, which to be fair is more nuanced than that and has also other purposes.
https://www.fsf.org/bulletin/2022/fall/copyright-assignment-...