Complex sentence: The DC-3s were reinforced by Convair 240s beginning in 1960; starting in 1965 the CV-240s became Convair 600s with Rolls-Royce Dart turboprops.
Split into simpler sentences: The DC-3s were reinforced by Convair 240s beginning in 1960. Starting in 1965 the CV-240s were upgraded to Convair 600s with Rolls-Royce Dart turboprops.