Yup, as usual Z availability is poor on Delta for confirmed upgrade at booking, and even for clearing as you get closer to the flight date.

Just make sure you get put on the airport upgrade waitlist if it doesn't clear before you check in!

I assume you're on DL273 which is showing J9S6I1 but Y0, so I'd think you have a good shot at clearing. Kind of surprised they haven't cleared some people to Z to open up more Y seats for sale as it appears to be oversold. Maybe you'll get an op-up and not even have to use the SWU at the gate.