National Bureau of Standards Reactor (NBSR) uses a refueling cannon system in-between 38-day reactor operation cycles. The refueling canon system is an electro-mechanical system to transfer fuel elements between the reactor vessel and the storage pool. The system was in service since the initial construction of the reactor, during which time few upgrades had been made. Since the last upgrade, the system operating interface was an electrical panel that consisted of pushbuttons and lights that demanded frequent care and maintenance. The instrumentation was controlled by an analog relay logic circuitry, superseded by Programmable Logic Controller (PLC), which was only capable of basic instructions, preventing implementation of any complex control logic. The manufacturer support for this PLC modules and hardware had been diminished for many years. PLC configuration and programming software required an outdated DOS PC platform and communication hardware, that made it infeasible to configure the PLC using modern computer system interfaces and utilize the configuration software to make changes as required. These issues, along with new operational and safety requirements and the need for a modern, high performance user interface led to a decision to upgrade the Refueling Canon control system with a modern PLC and PC based Human Machine Interface (HMI). The new control system was designed using Industry standard, modern PLC hardware for easy maintenance. PLC software application was implemented as a well commented, readable and modular program code that uses function blocks for repeating code to simplify the maintenance and future changes. Control logic and operating conditions from the existing controller were reverse engineered and implemented precisely in the new program logic. The upgrade produced four main benefits. First, a computer HMI interface was designed and developed in accordance to current industry standards. Second, the HMI was designed to include all the operator procedures, with automatic guidance and error preventing features. Third, engineering safety features were introduced, that define safe equipment operation in case of power failure or PLC power cycling and for the shutdown state of the system. Finally, the old control cabinet experienced numerous changes over the years, resulting in tangled, hard-to-trace, unlabeled wiring, leading to difficult maintenance and troubleshooting. Thus, the upgrade included a total redesign and rewiring of the control cabinet electrical circuitry. In this paper, we discuss our experience in reverse engineering an aging system and the design, verification and validation, testing and installation of the replacement system.