It’s bait and switch to community developers who contributed free labor to a for profit company for what is now either a fork or a more restrictive license
The license change only affects the code written in the future and now people can change their mind about contributing.
That seems fair to me.
Maybe you think that morally the license should never change but there is no clause in the license to prevent changing the license, so that would not be a reasonable expectation.
Reading the other comments the switch does make more sense, if you want to freeload off of the work redis has done for the project then you’ll have to join whatever community remains on the forked version, and anyone who cares about this kind of stuff should probably understand what’s permissible in the previous license, which clearly includes it being switched out like this