My statement is this regarding the OP:
Their new system is actually better than attempting to tie 110V units via relays or other divorced methods to a UL listed alarm panel, as the circuit and backup battery for the units are now supervised via the panel. Also, the detectors (depending on manufacturer) generally provide some sort of indication as far as detector sensitivity goes, as well as most likely went from ionization to PE for detector type.
It's also not 100% true regarding the 110V requirement, as nowhere does national code dictate the only smoke detectors allowed for this purpose must be 110V. The only requirements are there be 1 smoke detector on each floor and 1 in every sleeping area, with a detector immediately adjacent to the sleeping area(s).The detectors, however, are required to be hardwired as well as generate a tandem ring. Any variance that does not allow LV units for the purpose of primary fire detection is entirely a local mandate.
The typical arguement most AHJ's provide is if the alarm system isn't maintained or monitoring service discontinued, that the fire devices still function as intended. I've (unfortunately) had to argue the position with a few electrical inspectors and fire marshals, since barring a panel blowing up, the maintenance and backup provisions are no worse than 110V units.
The only functionality that would need to be maintained, barring an AHJ's objection, would be tandem ring and appropriate sounders in each smoke detector. Code does not dictate if the fire system needs to be 110V or low voltage. If push came to shove, I'm sure the AHJ could be brought in to verify compliance in regards to the certificate of occupancy, and honestly, the new setup meets or exceeds the original design criteria from 13 years ago, let alone 2012.
