I currently have an Ultegra Di2 6870 SS (short cage) derailleur fitted but I'd like to increase the size of my cassette. Unfortunately, to do this, I'll need a GS (medium cage) derailleur. I'm assuming (and stand to be corrected) that the main part of the derailleur is identical on both and that the only difference is the length of the cage plates? If this is the case, can I simply purchase and swap the cage to turn my SS into a GS? At £147 for a new GS I'm looking at other (cheaper) options!