Hi Doug.
Sure, you can probably do it. Sometimes there is some fitting required when installing a new barrel or cylinder. And there is the cutting of the loading port into the frame. Doable if handy with tools.
You said you were going to use a conversion kit. And since you mentioned a loading gate, I'm assuming that would be a Kirst Konverter conversion cylinder (I don't believe R&d are available with loading gate). The loading gate is attached to the conversion ring of the cylinder. As mentioned, you have to open up the frame on the right side if you want to be able to load from the rear with a loading gate. So, to change back to C&B, you just take out the Kirst cylinder and attached conversion ring and put back in the percussion cylinder. No changes to the hammer since the firing pin is in the cylinder. This is assuming you leave the barrel unlined (which requires hollow-base bullets to be accurate in the .375-ish bore). Or, since you are talking about a new barrel, you could line that one for standard .357 bore and leave the other one for percussion. Or there is also the option of using healed bullets for the larger bore.
On the other hand, perhaps you are talking about the conversion kit that screws the conversion ring to the frame. I know that one is more involved. I'll let Raven or some other conversion guru comment on that one.