I'm not a machinist or a gunsmith, but I stay at a lot of Holiday Inns------
I know a couple really good machinist/gunsmiths, custom gunbuilders guild guys, friends who have done work for me building single shots. the question of reshaping octagonal barrels has come up since I've had it done on single shots I use in ASSRA competition.
A lot depends on how the original barrel was handled, the steel bar has manufacturing stresses that have to be relieved, each step of machining, exterior as well as drilling, rifling, and reaming can add more as the bar is turned into a gun barrel. experienced gunsmiths know all about this and know how to normalize the stresses that can cause a barrel to warp.
Heat from firing can cause a barrels inherent stresses to become active shifting impact point. ( this may not be too big an issue with our muzzle loading BP rifles) thats why most precision target rifles have short fat very stiff barrels. Barrel vibration harmonics are another factor and I have no idea how having a long barrel pinned in several location affects this.
I suspect that planing down an octagonal barrel from straight to taper, might create or reactivate normalized stresses if not done properly. It seems to me there must be a process to avoid creating them, or normalizing them afterwards.
I think if I were considering doing so myself I PM someone like Don Getz or contact one of the good custom barrel makers like Rice, Colraine, or others and ask their advice before proceeding any farther.