Converting a conversion? There are some that convert percussion conversions back to the original flint, so why not? I admit I've never encountered such a pinfire conversion, but I understand your logic. I've searched for a variety of conversions, always amazed at the skill of the work involved. I am most impressed by a conversion of a Horsley pinfire to centrefire, where the standing breech was strengthened by the addition of a steel slab, and re-engraved to hide the work (the mis-matched right hammer is surely later work).

[Linked Image from i.imgur.com]