As one who goes back and forth between a Euphonium (using a 51D mouthpiece) and a Mellophone (using 6V mouthpiece - I know it's small, but I love that sweet "mello" sound it gives), I can tell you it's not nearly as difficult as it seems like it should be. As someone mentioned earlier, I have also found it extremely important to put in at least a few minutes on each horn, each day. Although my "primary" practice horn may change from day to day, I always leave time to play the other at least a little. Unlike some of the others, I started with the bigger horn, and later added the smaller one. Like them, I have seen (ok, heard) a definite improvement in tone. I also learned to much better stay in tune from note to note, without using the slides. And I can play killer pedal tones on the mellophone... much to the dismay of some with a lot more experience than I have on the horn.
This is Phantom... the Buick... you know they wouldn't mess with it if they couldn't do it right.
I can't wait to see this part of their show.