morceau symphonique
Good points so far made by uieuph. I have a few suggestions regarding both your new mouthpiece, and Morceau Symphonique. As far as the mouthpiece goes, I agree with uieuph, you should definitely take a bit of time to make yourself used to the new mouthpiece. Take plenty of time over the next few weeks and months to go over old etudes and warm up drills/exercises that you had been taught on your previous mouthpiece. You want to essentially take the music and exercises that were very familiar to you on your old mouthpiece and play them all as a way to get used to your new mouthpiece. I mean, just playing on your new mouthpiece you are going to get used to it, but we want to make the learning curve as short as possible.
Regarding the Morceau Symphonique. It is a piece that is just about as standard as it can get for Trombone and Euphonium players. While this piece offers some challenges for euphonium players, it generally is a very satisfying piece to play. I do agree with uieuph on the point that an adjudicator might be a little bit annoyed to listen to high schoolers playing pieces that have technical requirements that are a bit beyond their current ability, and might be more satisfied to listen to a student who is playing something that is more appropriate for his/her skill level. This is not to say that it is not a good thing to push yourself to develop your abilities (including your comfort in the high range), but as uieuph said it is not simply enough to just be able to play the high notes. It is important to be able to effortlessly leap from a middle E natural to a high C sharp (a major sixth), slot the note and play it with good tone and intonation, and be able to do it while in a concert situation that will probably have you playing a bit more tense than normal, increased adrenaline... well, you get the picture. While many of the things that I have said so far may have sounded negative, I don't really want to discourage you from playing this solo. I just want you to be able to make an accurate decision as to whether you should be playing the Morceau Symphonique, or picking another solo to play that is more appropriate to your level. If you feel that you can comfortably handle the challenges that are provided by this solo in a concert setting, then I would absolutely encourage you to give it your best shot. You didn't mention whether or not you are currently taking private lessons. If you aren't then I would really encourage you to, especially since you said that you are playing this as a solo for contest.
Uieuph mentioned that you should iTunes the solo... well... lucky for us, we have an absolutely phenomenal recording available to us on this website. It is a wonderful recording of Dave Werden performing it with the Coast Guard Band. Here's the link:
http://www.dwerden.com/DW-recordings.cfm
Good luck with the solo, and enjoy the recording!
Gregory E. Lopes
Euphonium player
US Navy Band Great Lakes
US Navy Music Program, 2009-Present
Besson Prestige 2052