Complex sentence: As the airline grew it needed larger planes, the firm decided to buy 5 DC9s in 1965, the Convair 340s were also changed from piston power to turboprop.
Split into simpler sentences: As the airline grew it needed larger planes. The firm decided to buy 5 DC9s in 1965, and the Convair 340s were also changed from piston power to turboprop.