It really is as simple as plugging the servos into the new receiver. If the planes are really old, they may have the "old style" Futaba plugs which will require you to solder on a new connector. Also, be wary of the old servos. If the planes were stored outdoors, thermal cycling and corrosion have taken their toll. You also should check if the old servos can use 6v power if you're going with a Spektrum or JR system since they have a history of browning out with 4.8v.