Well really the people change what notes that instrument plays. Say you had a really high-sounding instrument, to avoid writing loads of ledger lines people would write its music an octave lower, thus "changing" its pitch. Also, sometimes it would be easier to have fingerings on different wind instruments the same, so that it would be easier to learn a new wind instrument when you already know another. For instance, on the different clarinets they "changed" their pitches so that their fingerings would be the same, ie, they would have the same fingering for "C" although each of their "C"s sound different.
Yeah. I wouldn't know.