If it is the very early model with the bolster, just for get it, not worth even thinking about. The later ones with the drum and nipple are relatively easy to switch, but require a little bit of metal work. The breech system was designed to be very strong, but trying to remove the parts in the wrong order will destroy the barrel. The drum is cross threaded through the barrel wall into the breech plug. Then an opening from the bore into the drum was drilled with a long drill from the muzzle. For that reason, taking it apart and being able to return everything to it's original position is difficult even for some experienced gun smiths.
The other way, easiest for most, is to cut the drum flush with the barrel and then drill and tap the stub of the drum in the barrel with a touch hole liner. install a flint lock and go shoot. The problem with this method, requires a little finessing to get the touch hole at the proper elevation to the pan. At one time, I bought several of those Spanish Maslin style flint locks for $20 a piece new. Those days are long over. If you can get one from Deer Creek new, that is the way to go. As dragetbill said, the gun will still be worth very little. However, for folks who are required to have flintlock ignition, like Pennsylvania's three week Flint only season after Christmas, it can be the cheapest way to get a flintlock rifle. In any other state, I would see little reason.