Change of mouthpiece? A thorny subject if ever I heard of one.
My teacher suggested that whatever mouthpiece (instrument) one changed to, you sounded the same after a fortnight! Possibly true. My feeling is , get a mouthpiece that does most things quite well (sound, range, flexibility, endurance etc) and the rest is up to you and the ammount of practice you can afford.
If you are naturally very good at one aspect of playing (perhaps you make a huge sound, or can play high easily and freely) then you could chosec a mp that favoured the area you wanted to improve without losing too much of your strong area.
In the old days dozey policemen were given lively horses to wake them up and aggressive coppers were given sleepy old mares.
Basically, don't get bogged down, it's only a mouthpiece.