For nearly 20 years, DoD has had only one narrowband voice algorithm called Linear Predictive Coder (LPC). It is used in the Advanced Narrowband Digital Voice Terminals (ANDVTs) operating at 2400 bits per second (b/s). Currently, 40,000 ANDVTs have been deployed by the Navy, Army, Air Force, Marine Corps, and special government agencies. DoD is currently planning to develop a new narrowband voice terminal called the Future Narrowband Digital Terminal (FNBDT), which features a new voice processing algorithm called Mixed Excitation Linear Predictor (MELP) operating at 2400 b/s. In the future, LPC must interoperate with MELP. Therefore, it is essential to develop a technique that enables MELP and LPC to interoperate, so that secure voice service among narrowband users will not be interrupted during the transition period. Although LPC and MELP could interoperate through the age old tandeming method, resultant speech degradation would be very severe because the bit stream must be converted to the speech waveform, which is re-analyzed and re-encoded. Therefore, NRL investigated an alternative interoperation technique, called transcoding, where speech parameters, such as pitch, amplitude parameters, and filter parameters, are directly converted from one to the other vocoder. This report documents the computational steps required for transcoding and their theoretical basis. According to formalized tests, transcoding did not degrade speech intelligibility in comparison with LPC-10.