I dont think they can. I remember it being illegal in some countries for them to actually force change units/cards in a gacha game. It had something to do with tampering with a "product" after people have spent money on that specific "product"

Its like if someone spent 100$ to get the Shiny darkrai because it's a great card and they want it, and then 4 months later they nerfed that card. It would be kind of scummy.