If I understand you correctly (I haven't really been following this), they changed the contract and are trying to retroactively collect license fees for installs done prior to the change in contract? I don't think this is legal. When you change a contract, it's on a go forward bases. It will be interesting to see how this plays out. It's definitely a money grab. If it's deemed illegal, i.e. fraud, I hope there is jail time. Gotta send a message.
Did Unity recently get acquired, new investors or new management?