Most of the literature on implementation of new pay systems is filled with disasters of varying degrees. Add to that the DND penchant for running things into the ground (ships, vehicles and aircraft) and we find ourselves with CCPS, first released in 1977 (!) and RPSR, first released in 1994 as IRPPS as the payroll systems.
Last I heard, the as-is payroll system (Reg and Res) is being fully mapped, interdependencies documented (Guardian, PENMod, various CRA systems, DRMIS, FAA, NDA...) for the deliberate conversion to a COTS product.
But in the interim, the in-service 49 and 32 year old systems require ongoing maintenance to keep them operating and keep them compliant; upgrades and changes for new benefits create additional demands on a shrinking base of technical personnel.