Len's advice is obviously sound. And I would add a word about mpieces in general - you say you've been advised to change; I'd have needed a very very very very VERY good "why" before I changed absolutely ANY aspect of chops when I was playing. Its a minefield - and often the best advice is "if it ain't broken, don't fix it".
Doesn't sound come from the diaphragm support rather than the mpiece? If you've got the range, it would suggest to me that the mpiece is ok, ish, and the sound needs to come from somewhere else? If you sound decent at the top and bottom, can't you just get a bit more air through the mpiece you've got?
In any event, remember that a new mpiece feels like one of Saddam's "desert water pipes" when you first get it, and a couple of weeks later its like blowing down a weasel - they close up on you!
Biggest advice - just don't get a "thing" about mouthpieces. That way torture lies!