Originally Posted by
RealHJ
You need to use GUCs for GUC upgrade inventory (to the last seat…usually). If you want to use RUCs, look for flights where it says RUC space is available (called “upgrade certificate” on the website, without the “global” in front). Be prepared to look and keep looking across many months and keep doing so for weeks until (and if you ever) come across the rarest of unicorns, that is Delta RUC upgrade on A330/350/767. It can happen, but you need to be very lucky to find it.
I actually found RUC availability to HNL pretty easy just a couple of months ago. It was very easy to find the MSP-HNL segment in D1 that way and a little harder to find the HNL-ATL that was but I pulled it off.