[csande17]

This only really works if the contributions were made under a copyleft license like GPL. With MIT, it's perfectly allowed to rugpull like this so long as you bury the original copyright line/disclaimer/etc somewhere in your app's equivalent of chrome://credits.

[Gormo]

No, with MIT, you are only releasing your subsequent modifications/derivative works under a new license. You can't retroactively change the license to anything that is already MIT.