A german games magazine did ask a lawyer now. To summarize: Changing the rules on already published games is only possible if the "specific rules of changing" are already a part of the contract and don't constitute unfair practices. Since it is highly unlikely that such terms would be already included in old contracts that Untiy used up to now, games companies would have excellent chances in courts. Also, since Unity has a dominant market position together with Epic (oligopoly) this would even bring antitrust authorities into action.
And at least in Europe getting the relevant data for their installation estimations would be either expensive or impossible with current privacy laws.
In other news: Unity pedaled back and said it would rethink their approach. Needless to say they want and probably need to increase prices somehow. We'll see what they come up with, but I think they have got a lot of bugs reports for their current plan 😉