CMP Rule 6.1.2 Service Rifle Modification:
The dimensions of the rear sight (internal diameter) and the front sight post (width) MAY vary from standard military dimensions. The rear sight hood diameter may not be longer then 0.70" or larger than .050" in diameter. The rear sight aperture MAY have a fixed, non-adjustable round, square or rectangular aperture insert. A corrective lens or multi-focal lens system may be inserted in the rear sight hood.
Basically what that means is you change the inside of the rear sight but it has to meet the outside dimensions of the military rifle sight.
NRA service rifle rules are less restrictive then CMP rules, When the rules are in conflict the CMP governs, or you have to abide by the CMP rules.
You want your "service rifle" set up per CMP rules. If you shoot high power service rifle you're going to want to work on your distinguished rifle badge, which is a CMP Match or other CMP matches such as the President's Hundred.