The -7's were original equipment on the B727-100 ,the original B727 ,the -9's were original equipment on the B-727-200 series.
EAL had the brilliant idea of paying extra money to have Boeing modify their B727-200 or order to the -7's in order to have a common engine for the DC-9-30,B727-25,B727-225 fortunately only 25 EAL were the lead sled .The original configuration called for 118 pax on the B-727-225 used on domestic short to medium runs.
So the -9's used for pax loads of 175 pax ,on long hauls,and in tropical temperatures, would certainley be to a safety conscience crew member, another lead sled.
EAL did upgrade to the -15's then the =17's for the South American runs.