Ken,
I’m guessing it was a combination of all.
I have one rifle that clearly shows that the barrel was cut back one inch from the rear while it still had a full length forearm, because all the barrel underlugs were repositioned one inch forward.
Then at a later date, the barrel was chopped off six inches from the muzzle and the forearm cut back to a half stock configuration. I’d guess at this time the flint lock was also percussionized.
One can only guess why the one inch was cut off the rear of the barrel early on. Of the totally original flint guns I’ve seen, the wood and metal in the lock area seems to fair much better condition wise than the percussion guns do. If the touch hole was simply burned out, it would have been easier to re-bush the hole than re-breech the barrel, but something must have gone haywire back there.
The second whacking taking 6 inches off the muzzle and trimming the forearm back I see as sporterizing, to shorten and lighten the gun up a bit. And also to modernize it with a percussion ignition. This particular Lancaster rifle also had a buckhorn rear sight put on the barrel and a tang sight gizmo installed, so it might have gone out west at some point.
All this whacking and chopping on Kentucky’s seems to have been fairly common. In fact, I’d say original full length barrels today might well be more of a minority than a majority on well used guns.
John