Totally depends on when the building was permitted for occupancy by AHJ. Code would follow the building for as long as the occupancy and use is the same, has not changed since the building was orginally permitted. The required code for the building would be grandfathered for as long as it is being used for the same purpose under Texas Administrative Code Rule 34.616. The building is not changing purpose or use, therefor is not required to adopt new code until the building is being repurposed for another occupancy use or the building is being designed or is being constructed differently, changing the required fire protection, new design to current code, needs to be re-permitted and approved by AHJ.
Replacing system that has same occupancy and use with same or similar with respect to fire would be standard repair service for this building and the adopted code.
Texas Administrative Code RULE §34.616 (5) Fire alarm system equipment replaced in the same location with the same or similar electrical and functional characteristics and listed to be compatible with the existing equipment, as determined by a fire alarm planning superintendent, may be considered a repair. The equipment replaced must comply with the currently adopted standards, but the entire system is not automatically required to be modified to meet the applicable adopted code. The local authority having jurisdiction (AHJ) must be consulted to determine whether to update the entire system to comply with the current code and if plans or a permit is required prior to making the repair.