I've been playing on my Conn 3 mouthpiece ever since i started playing. I just recently (recently as in yesterday) got a new trombone but the bore is larger so my lovely Conn 3 doesn't fit. Can i buy that same mouthpiece for a larger bore? or are the numbers different with different brands? help me! i need some guidance here!

thanks a lot.

peace.

http://ibowtie.com/xcart/pages.php?pageid=4
here is a pretty comprehensive mouthpiece comparison chart.
sounds to me like you're looking at something along the Bach 6 3/4 size?
if that's the case, you could get a large shank Bach 6 1/2AL .. many players do well with that mouthpiece.
Ask your teacher, it may be time to go a bit larger since you're on a larger horn now. Maybe a 5G? very popular combination, but a teacher who can see you will make better recommendations.
Or, if you have money to spend, contact Doug Elliott at www.dougelliottmouthpieces.com
He'll make you just about any size mpc you could want. I have used his mpcs for a long time and would hate to use anything else.