I am no expert but it makes sense to me to do as much cardio as possible, lessen the calories and protein intake as said here already, cut down on carbs also, but still make sure you eat carbs for endurence with the cardio. Also, I think two to three times a week doing high reps full body workout with 3 sets of 20-35 reps would tone the muscles and make them become smaller because the muscles would become smaller to support physiological systems that are for endurence instead of intensity. I might be wrong though, definitely go to an expert.
I would also be worried about when the muscles start shrinking that they may lose proportion, maybe the high rep routine would take care of that...
It just occured to me also that maybe doing a workout once a week of high weight like you are used to and taking the weights down week by week may also be good.
Last of all I am wondering what happens to the skin when the muscles start shrinking, does it become flabby? Anyone know anything on this subject? it has always interested me. I heard once that Sylvester Stalone once radically lost size on purpose for a movie he was doing or something and he had a special diet and routine etc...