Haven't heard this, but if true, they're probably upgrading their CFM56-B22 engines to either a -B24 or -B26 rating for more performance and payload lift. From what I understand, it's essentially a re-calibration and re-setting of the existing engine controllers, along with some other minor modifications, along with $upporting paperwork/documentation (always big bucks).
They have about 190 of the -300s, and 25 -500s, so that 300-engine figure you mentioned equates to 150 aircraft. maybe they're only doing the 150 newest -300s and -500s in the fleet.
Where'd ya see/hear about this?
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