There are quite a few changes that have to be made. Usually, the cylinder window is opened up to accept a larger oversize cylinder. If you are changing calibers, obviously, a new barrel will have to be fitted. Even if you aren't changing calibers and go with the larger cylinder, the barrel will still have to be faced back to allow for the new cylinder. The hand will have to altered to account for the 5 shot cylinder and not a 6 for the timing to be right. Most of the 5 shots will also have a bolt bearing block of some sort to take the "play" out of the bolt and tighten up the lockup prior to lineboring the new cylinder. They usually do an action job while they have it apart and then the caliber designation should be changed if you went with a different caliber. Sights are always a nice upgrade. The parts are usually made from scratch in the shop from these smiths that do this work routinely. I know they used to have "predrilled" cylinders that needed to be chambered for sale but I've heard some pretty bad reviews of some of the ones that were being sold through brownells. Some may have been ok but I don't know. There may be parts I'm forgetting but this is a start.....
