It's also going to depend on the size of the breech plug. I just got a barrel back from him that I was going to go up from .50 to .54 and he wasn't able to do it because the breech plug was too small. Ended up getting it sleeved down to .45 with a fast twist. All together it cost me less than $200 including shipping to and from. You're going to have to talk to him and tell him what you have and what you want to do. He will point you in the right direction.
In conversations with Mr. Hoyt about TC barrels specifically, he told me that ‘usually’ he could go to 58 caliber rifled bore or 60 caliber smooth bore with a 1” across the flats barrel, as long as screw holes and dovetails weren’t too deep. On other manufacturers’ barrels, basically the same factors plus the breech plug, as previously mentioned.
Your best bet to contact him is to call first thing in the morning before he starts up the machines in his shop.
