If I recall correctly, the FCC changed the rules slightly a few years ago. It's now permissible for an installer/maintainer to replace an OEM antenna, by another antenna which has a similar pattern and equal or lesser gain... the new antenna does not have to be formally certificated along with the radio.
Manufacturers are supposed to provide information about the highest antenna gain with which the radio/system has been certificated, and suitable replacement antennas, so that installers/maintainers can know which antennas are legal to use with the radio.
The rule change I was thinking of, was made in 2004:
(4) Any antenna that is of the same type and of equal or less directional gain as an antenna that is authorized with the intentional radiator may be marketed with, and used with, that intentional radiator. No retesting of this system configuration is required. The marketing or use of a system configuration that employs an antenna of a different type, or that operates at a higher gain, than the antenna authorized with the intentional radiator is not permitted unless the procedures specified in Section 2.1043 of this chapter are followed. They did not remove the "unique connector" requirement in 15.203.