As far as I understand the law,
if you call it in (re-verify the gun, whatever),
they will change the classification to restricted.
If the gun is registered as non-restricted
and you put a folder which makes the folded lenght
less than 26" without changing the classification,
then it becomes illegal.
(But I was wrong so many times before...).
+1 to what BabySeal is saying about
permanently attaching a muzzle device.
Well, I think that the same idea may be able
to work at the other end of the gun too.
Somebody was posting long ago about a genius idea
about welding a chain link (or similar thing)
on the trunion of the folder, if you are missing few milimeters.
Would that legally work???