After watching and reading some of the promotional stuff for The Marvels, I think I understand why they changed Kamala/Ms. Marvel's powers. It looks like they wanted her powers to be connected to Carol/Capt. Marvels so they could do the whole thing with them and Monica/Photon (or whatever they're using for her superhero name) switching places when they use their powers.
I think it's partly that, partly that they wanted something more novel than shapeshifting/stretching powers. Also, I suspect that having her powers tied to an alien artifact is meant to tie in eventually with Shang-Chi's Ten Rings, sharing some common origin that will eventually come out and connect to other movies as well.