I don't know if Delta has rules for gate agents or gives them great leeway.
We were upgraded on L fares JFK-FCO last year in a strongly oversold situation (the previous night's flight had apparently cancelled due to weather).
The gate agents made a careful evaluation of the load and seem to have upgraded based on (a)Delta status and then (b) fare level. We got upgraded based on Silver status; the folks around us we talked to were either leaders of tour groups or higher fare. Maybe this was possible because they knew far in advance that they faced this situation.
I am very surprised to hear that gate agents do not attempt to reward either status or fare level in upgrading. It is not really that hard to do unless the plane is already running real late.
Count your blessings. We are in steerage to Athens Wednesday with little hope; but the fare was
so good... maybe Tuesday will cancel? (slap my wrist; I should not wish so much disruption on strangers just to improve our chances for an operational upgrade....)
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"Service" should be a noun, not a verb.
Delta Flyers' Guide