The current system works as:

If Z is available, you get confirmed to business with a PMU/SWU at time of booking. Otherwise make sure you get on the waitlist. This should also appear in your reservation online as a line for each flight showing waitlisted for each segment that you are trying to upgrade.

If it does not clear by day of departure or when you check in... make sure you get to the airport early and request to be put on the airport upgrade standby list. This works similarly to the current upgrade list and your name should be visible on the GIDS. Z is not required for this upgrade to clear. As long as there are ANY seats in business, people on the gate standby list will clear... and you should clear before any NRSA are given the seats.

Also to be safe take a copy of your SWU numbers with you to the airport in case the agent can't see it for some reason.

NOTE: Use of same day upgrades with SWU only applies to Delta metal. If you are on the Z waitlist on a KLM flight it has to clear and be reticketed before you check in since they are unable to process the SWU at the airport... if it doesn't clear you'll be stuck having to use miles at the gate if you want to standby there.