The 5 cell packs will work just fine. The servos will have more torque and a bit faster speed. The battery life in flight is a bit shorter with the same size from 4.8 to 6 volts, though. Meaning, if you got 5 flights out of the 4.8 you might get 4 our of the 6 volt packs (just an guess, not based on actual fact). You probably will also need a charger that will charge 5 cell packs to get them fully charged.

CGr.