I always go online with my booking and change my seats 72 hours prior and they're always available.
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FIFOSuit when you make the booking do you tick the check box " complimentary upgrade request " on the passenger details screen ? or simply leave it and change seat inside timeframe ?
This is what I read on Delta.com
DELTA COMFORT+ MEDALLION UPGRADES.
Complimentary upgrades are obtained by first selecting a Main Cabin ticket. Then, on the Passenger Information screen check the box for a complimentary Delta Comfort+ upgrade. You will be notified of your upgrade via My Trips and email based on availability.