IF the BP gun is set up with a nipple drum that will remove then it is an easy job that will cost about $150. You have to buy a flint lock that is the same plate as the percussion lock. You will probably have to take a bit more wood out of the lock area for the flint lock but this will not hurt if you want to switch back to percussion. Then you have to order a touch hole liner the SAME threads as on the nipple drum and screw that into the barrel. When you switch back and forth you remove the touch hole liner and flint lock, install drum and percussion lock, and when ready to switch back you do the exact opposite. I have a 32 long rifle set up like that.
can help you with all you need and if you give them a call, tell them what gun you have if a company made one they will tell you the exact parts you need.
As to CVAs, some of them can be done as to switching BUT TCs cannot that I know of.