Complex sentence: All Group II aircraft have had their 1960s vintage computer-processors replaced by a mission computer with the same functionality that used more modern computer technology, referred to as the GrIIM RePr (Group II Mission Computer Replacement Program, pronounced "grim reaper").
Split into simpler sentences: All Group II aircraft have had their 1960s vintage computer-processors replaced by a mission computer with the same functionality but built using more modern computer technology. This is referred to as the GrIIM RePr (Group II Mission Computer Replacement Program, pronounced "grim reaper").