Bobby Hoyt rebored and rifled an antique barrel for me. First rate job.
If you just want to bored to a smoothie, just about any local gun smith that has a lathe can do it. While it normally involves removing the breech plug and reinstalling. A machinist told me it can be done without even removing the breech plug. It requires drilling out the rifling and then reaming down to within 3/8 inch of the breech threads. I would not go more than perhaps 50 cal out to either 54 smooth or 28 ga, (55 cal) We think in terms of common calibers and gauges, but if the smith needs to order a reamer a standard SAE size may be cheaper to obtain 9/16 = .5625 and 17/32nds = .531.) If the smith already has a reamer, the costs may be less.
I have often thought a smith could make a fair business reboreing old factory muzzleloader barrels. But the cost and time involved run the expense up to just about a new barrel