Are you still using stock electronics?
If so the HS-55 won't be compatible with them with out a mod. If you want to use the 55 with stock electronics you will need to convert it to a 4 wire servo.
Or you could go with a new ESC and Rx and then you can use the HS-55 with out any mods.
Either way you might want to pick up the TL servo saver for the HS-55 unless you want to grind down the 55 output gear into a hexagon shape to fit the stock servo saver.