It is possible to change the cage on many Shimano RDs. I have not tried to do so with a 7900, but you may be able to put a long cage on there from a different derailleur. Taking the cage off is not too hard, but putting one on often takes three hands plus a vice to hold it all in place and screw it down. However, as mentioned above, the Ultegra 6703 is a very good piece of equipment, and that would be my first choice.