Aser,
I posted earlier in this thread that I knew there were growth plans once upon a time to add a 5th blade when the MGW grew above about 32,000 lbs. That might still be true. I do know that the rotor lives with the 4 bladed head are very long (unlimited in most cases) so there is no reason why the rotor needs any changes now.
The transmission AD is undoubtedly due to a casting problem that was discovered by the Sikorsky testers, who then told the FAA to be sure that the internal Sikorsky service bulletin was adhered to. I would bet the parts are free to all customers, as the warranty is surely in effect for all aircraft.