Note the position of the hardest GPL defenders in this Github thread was that:

a) You need to have absolute 100% of the code ownership/copyright in order to change the project license. This is not true and there have been precedents where a valid and successful relicense was done with much less (if I remember correctly I learnt this from a comment somewhere hidden in that same Github issue)

b) The other project contributors would not necessarily agree with the relicense. Which was not the case: the most prominent contributors showed up and explicitly gave the author ownership (or free license) over their small parts of the code.