This paper examines a comprehensive set of options for upgrading legacy avionics systems in tactical aircraft. The options have been developed as part of study involving the avionics system of a current military aircraft. Multiple options are considered ranging from minor changes to the legacy system to complete replacement of computers and communications buses with commercial off-the-shelf (COTS) hardware and new software. The pros and cons of each potential solution are described and guidelines are given for choosing the appropriate solution for a given program. The results of this study may be used when evaluating upgrade options for a wide range of embedded computer applications.