DI2 help

[oliver1850]

I have no experience with the stuff and am in the dark. Had to drive 15 miles and pick up a pair of German tourists today, one who had snagged a stick and destroyed his rear derailleur. It's Ultegra and I assumed it was 11 speed but stupidly did not check the model number of the RD. After checking I find that Shimano has 12 speed Ultegra which I didn't know existed, would have checked the model number if I had known. He needs a GS cage because he has a 34 large cog. Are 11/12 RDs functionally equivalent? We are going to be looking for whatever works and is locally available.