You don't want to require something that would be considered retro-fitting.
Good point but I think requiring the gaurds during a unit replacement can justified through the current code lanquage.
EQUIPMENT, EXISTING. Any equipment regulated by this code which was legally installed prior to the effective date of this code, or for which a permit to install has been issued.
Once the equipment is removed it is no longer existing and the installation of new equipment is required to meet code.
Charging lanquage
301.1 Scope.
This chapter shall govern the approval and
installation of all equipment and appliances that comprise parts of the building mechanical systems regulated by this code in accordance with Section 101.2.
Mandatory general requirements as listed in Section 304
304.1 General.
Equipment and appliances shall be installed as required by the terms of their approval, in accordance with the conditions of the listing, the manufacturer’s installation instructions
and this code. Manufacturer’s installation instructions shall be available on the job site at the time of inspection.
Specific mandatory requirement
304.10 Guards.
Guards shall be provided where appliances, equipment, fans or other components that require service and roof hatch openings are located within 10 feet (3048 mm) of a roof edge or open side of a walking surface and such edge or open side is located more than 30 inches (762 mm) above the floor, roof or grade below. The guard shall extend not less than 30 inches (762 mm) beyond each end of such appliances, equipment, fans, components and roof hatch openings and the top of the guard shall be located not less than 42 inches (1067 mm) above the elevated surface adjacent to the guard. The guard shall be constructed so as to prevent the passage of a 21-inch-diameter (533 mm) sphere and shall comply with the loading requirements for guards specified in the International Building Code.