Couple things...
Since it is DL metal, you'll have to have the ticket endorsed over to DL in order to have the upgrade processed, which can be done no fees or anything if you get the right agent who knows how and is helpful.

M is eligible for upgrade with miles however there needs to be Z upgrade inventory available, and it won't get processed at the gate... so based on the J3Y0 it sounds like the flight may be oversold or they are holding Y seats due to possible other irrops from the big storm, so mileage upgrades will not clear.

SWU can be applied and put the pax on the waitlist, at which point they would clear at the airport on the gate upgrade list. All that is required to clear for this is any available J seat. You don't need Z available except for the advanced confirmed upgrade.