Question: are we talking about conversions made back to flintlock, 20-21st century, or those made prior?
Question - Why does it matter?
If the item is owned by the individual - and if it is not illegal, does that person have the right to do with it as he\she deems appropriate.
There are some historically important sites where the law does protect the site - from some - but not all changes the owner can make to it.
Guns of any type or age are not on that list.
It is sad that some older guns are modified, however, it is not wrong or illegal.
State governments all over the country are conducting buy back programs for guns of all types.
These guns are destroyed, melted, crushed, sold as scrap iron.
Some of those are highly prized and sought after, but the owners of those guns have elected to turn them in knowing they would be destroyed. Did they have the right to? Yes. Sad, absolutely, but they do have the right. The question asked here in this thread - is it WRONG to destroy or change a gun just because it is old. Most of these have no claim to fame or historical importance - other than being just being old. Do "We The People" have the authority to tell an individual that they do not have the right to do with their own possessions as they see fit? Or do we allow special interest groups or government to make all our decisions for us? That form of government and personal infringement is not part of our rules for this country. That set of rules was laid down in the late 1700's - we call it our Constitution and along with the Bill of Rights attached, is the very foundation of our being.