Other than replacing the connectors on the leads, older servos work fine, as long as you stay at 4.8v. I found they don't work well at 6v. I bought a plane for the engine that I used on my Something Extra and put an OS on it instead, then when I went to fly it, I kept getting servo jitter. Found out the 5 cell pack I was using was causing it, so put in a 4 cell pack and jitter went away. I tried using 4.8v rated servos on another plane that I use LiFe pack for the RX, servos would not work at all unless I used a regulator in the harness.
So if you use older servos, just keep in mind they don't work well with higher voltages.