1) It is no longer retroactive. Existing games on the market using unity can continue distributing without their business getting immediately impacted.
2) It's no longer per install, it's now "new people engaging with your game each month" and the numbers are now all self-reported. Originally Unity was going to calculate installs on their end in an opaque way.
3) You can choose to just use a revenue share arrangement instead of per install. Since it's the lower of the two you can effectively ignore whichever one isn't favorable to your business.