MEDEVAC, IIRC, is the more standardized version of LIFEGUARD when that term was used. Previously when used together, LIFEGUARD was used when a patient or organs were being transported from one location to another, while MEDEVAC was used to transfer a patient.
Case in point was TWA800, which was a LIFEGUARD aircraft, as it was transferring organs to Paris... obviously, those organs never made it. They standardized on MEDEVAC a while ago (Can't remember what version of the P/CG they finally standardized it on though).
BL.