The Kanstul shop will do this for you (and replate the mouthpiece). But they need to know what to turn it down to. So typically they need a mouthpiece with the shank size/taper that you want, and then they can turn your other one to those dimensions. Last time I had this done a few years ago I think it cost me $50 (plus shipping).
Gary Merrill
Wessex EEb Bass tuba (DW 3XL or 2XL)
Mack Brass Compensating Euph (DE N106, Euph J, J9 euph)
Amati Oval Euph (DE 104, Euph J, J6 euph)
1924 Buescher 3-valve Eb tuba (with std US receiver), Kelly 25
Schiller American Heritage 7B clone bass trombone (DE LB K/K10/112/14 Lexan, Brass Ark MV50R)
1947 Olds "Standard" trombone (Olds #3)