This is a topic that concerned be, too. And good evidence of matchlocks being converted and updated to flintlocks exist archaeologically in the horde of surviving muskets found in the shipwreck of the Massachusetts Bay warship THE ELIZABETH AND MARY, sunk in 1691 at the mouth of the St. Lawrence River as part of failed a Massachusetts Bay effort to take Quebec City. Asked by Parks Canada about a dozen years ago to be a consultant to them, I found that the wood, leather & paper of the cold water wreck site preserved several dozen guns, among them one that was converted to the English lock. You could still see the long rectangular mathlock plate mortise to the rear of the flintlock plate. The musket was of the "fishtail" form. Othe muskets showed that they used recycled matcklock barrels that still retained the dovetail mortise for the missing matchlock flashpan, but the musket was new and used a flintlock. Amazingly, cartridges and leather cartridge boxes were also preserved! There may be something online about it. I have some images I will look through. The soldiers were from the Dorchester Regiment from Boston. All were lost in the wreck. I am, Your humble servant, Rich Colton