What I don’t get about this is how can Unity change their contract without renegotiating? Do developers and publishers publishing Unity games renegotiate released game contracts annually?

I would never sign a contract that allowed one part to unilaterally change the agreement so they could impose more fees without having to come to an agreement. And if you DID sign a yearly contract to continue publishing a game, that sounds like a recipe for Unity to hold a popular game hostage for renegotiation.