No, don't do replace it with 737, it has lower speed, it could possibly reduce schedules and has lower capacity. The 757 has first class and it failed to fill up in my experiment making lesser money than 727. The 727 stops production in 1984, replace aging 727-200 with 727-200 temporarily and in 1988.1 you can replace all old 727 with A320 (I did this way). A320 has 180 seats while 727 has 189 and same cruise speed