Sarg-

Nothing to get upset about. I've seen such guns from late in the transitional period.
I'd never do anything like that to a gun or rifle of great quality, as the value of the piece would get killed in the process.

Personally I'd prefer to run any surviving relic of the era as it was found. There's a lot to get from getting through all it takes to have a running pinfire long gun.

For your money, spend on a cheap central fire Belgian (or baikal or whatever).
Add long noses to the hammers and convert it to pinfire, keeping the central fire hardware.
I just don't see the merit of throwing the effort into hacking an original pinfire action with the only benefit present being able to shoot central fire cartridges.
The world is filthy with dirt cheap central fire long guns of all descriptions.
Something else to consider is that the fore-aft depth of pinfire action standing-breeches is typically shallower than those designed for the central fire system.
And if you aren't a good machinist it's going to cost quite a bit to have done - and you'll have something that *might* still be useful (consider proof) at the end of the job.

If what you want is an example of that moment in the transitional period - with both firing systems in place - just find an original one that's still intact.
It'll take a while to find one, and it'll cost a bit -- but you won't be throwing your money away.

It can be done.
I just don't see the point.
Wouldn't do it myself.
If the hermaphrodite action type is what you want, start with central fire and work back in time.

--Tinker