I posted this today on a discussion forum:
I don't see why collectors are disturbed when an old gun is altered. When it happens, their pristine examples appreciate. They should like that; if they fully thought things through they would encourage sporterizing.
If collector value were never lost through modifications, or damage, or rust or fires, or losing the rifle overboard, collector value would not exist, for all-originals would be plentiful. The value of any old military rifle would remain just where it started: pick through the barrel for one you like and give the man $89, or see if you can dicker him down a bit or get him to throw in some ammo.