Many nations today face the choice between procuring new aircraft or upgrading their existing fleet aircraft. An upgrade is frequently seen as a cost-effective solution to meet new mission requirements in a timely fashion. An upgrade allows the user to capitalize on technological advances since the development of the basic airframe. A key aspect of any aircraft program, whether an upgrade or an initial development, is the flight test phase. Flight test is the final stage where the new capabilities are evaluated for their likelihood to deliver added utility to the warfighter. However, given an avionics upgrade for a proven aircraft system, such as the F-16, the need for a flight test program is often questioned. "After all, it is only software" is a common comment. This paper will explore the need for, and benefits of flight test in upgrade programs. It will address the economics of testing, examine the limitations of upgrades, and touch on issues of incorporating new technology into existing weapon systems. Examples and lessons learned from actual programs either tested or currently under test at the 416th Flight Test Squadron, Edwards AFB, California will be incorporated. These flight test lessons can be easily applied to other procurement programs.