Yeah, I think that’s straight up illegal and I would simply refuse to pay.

If they can retroactively change terms, why can’t I, as a bonafide counterparty in that agreement? Maybe something like a 100% discount on runtime fees for days that ends with ‘y’.

Otherwise I could simply “retroactively apply” a 100% discount on my lease or new car purchase. Or for a better comparison, retroactively charge my employer for previous work, as I now decide to add a fee?

The correct answer and what all studios/devs should do: tell them to retroactively pound sand and ditch Unity for all future projects.