I may be wrong, but I didn't think that the authenticity of that particular document has been challenged -- only that of the two memos. Furthermore, there would be nothing damning (to Bush) about that letter in and of itself, since such routine orders and memos are sent all the time to service members when their name and date comes up for a physical. If I had a few bucks for everytime I have received some sort of military order or memo instructing me to do something that didn't apply to me, I could take my wife out for a nice dinner.
If, however, that document is also forged, then you are indeed correct that if the forger could be identified, criminal charges could indeed be brought on the basis of that order for a physical.