I think the problem is that you're having trouble adjusting between two redically different sized tip openings. The beechler has a tip size .083, compared to the Selmer's .067 That is a big jump, which is making the beechler hard for you to handle. My suggestion to you is to try a jazz mouthpiece in an easier(smaller) tip opening more comparable to the C*, like a Meyer 5M(extremely popular and great all over jazz piece for the alto) or an Otto Link in a 5-6 facing. not many crazy people(like me) use the huge facings on the alto, and especially not at your age, being a young and developing player

I don't agree with definition on this one. While adjusting between .067 and .083 might be a little of the problem I do not think that it is the whole problem. When I changed from my concert piece to my meyer metal (.072 to .105) I squeaked a lot too. However, I found that when I tried to play higher notes, middle notes, and from middle to high notes I squeaked. I consulted my lesson teacher at the time and informed me that it was an inconsistency in my embrochure. When I changed notes I was biting down on the top of the mouthpiece and that was causing me to jump about two octaves into a very annoying chirp/squeak. After I got this information I worked for about two or three days trying to fix the inconsistency in my embrochure and by the fourth day I had barely any squeaks at all. The point is that I believe your squeaking is attributed to a manipulation of some kind to the embrochure. Granted, you might have to take a little time to get used to playing on the more open mouthpiece. My advice is to try and see if when you squeak you are doing something weird with your embrochure and if you are then spend some time to iron it out. I wish you the best of luck and I hope you find out what is wrong. Cheers, Dark Eyes (unloved thesis)