This paper highlights some lessons learned in the developmental testing of the C-141 Global Positioning System Enhanced Navigation System (GPSENS) avionics upgrade. The authors planned and executed the flight tests of the GPSENS systems and learned many valuable lessons about retrofitting modern avionics into older aircraft. They found that commercial off the shelf acquisition strategies seemed much more suitable for hardware upgrades that were not heavily dependent on software. Overcoming limitations in legacy software products proved to be extremely difficult. Also, integrating modern avionics into legacy systems organic to the aircraft was often a hit-or-miss proposition. They learned the value of a high-fidelity engineering simulator and saw the results when the available simulator failed to properly emulate the aircraft in the areas of inertial systems, aeromodels, hardware-in-the-loop and electrical power quality. Finally, in validating a new Kalman filtered navigation solution they developed flight test techniques to improve a system that initially performed very poorly. Their lessons learned should serve as a guide to other engineering professionals embarking on similar development or test efforts.