Why switch from one of the greatest brakes ever (Shimano XT) to one that has continuous complaints. One (Avid) that is very temporamental...I can go on but what's the use???
I've heard two different stories on the adapter compatibility between Shimano and Avid. One LBS says, "yes, they are interchangable"...the other LBS says, "no, they are different".
Make it easy on yourself, upgrade to new XT's or keep what you've already got...