As caps became available it was probably "somewhat common" to convert existing flintlocks to cap - but since that would be a personal thing, it would depend on the owner.
Simply looking at a lock I have a difficult time accepting that all that are "advertised" as converted were in fact actually converted.
My take is - when percussion first started becoming wide spread, gunsmiths "didn't know" what a percussion lock was "supposed" to look like (since they didn't exist) and many early percussion lock plates "looked like" they were flint.
They may have been converted "before" they were installed in the rifle, just as well as after. OR they could have simply used a "flint" lockplate to build a percussion lock and at "no time" was it ever flint.
Here's a couple pics to illustrate what I am talking about. The builder was John Armstrong who made his own locks.
Here is one of his flints -
Now, looking at this next lock you might conclude it was "converted" - but it never was - it was just the way he built his percussion locks - just made the plate the way he "always had" but adapted it for percussion.
(no evidence that holes for the frizzen/spring were ever present and later filled - they were "never there", so was never a functioning flintlock and wasn't built as one)
If the rifle can be specifically dated to before the widespread of percussion you can say with fair certainty that it was converted, but otherwise it would be a guess at best unless there is considerable other evidence aside from the lockplate itself.
