Elites can call the Elite Desk 24 hours before the flight and request to pay for an upgrade (similar to the way it worked years ago for domestic flights). As mentioned earlier, seats do need to be available and it is often hit or miss in regards to success rates. This can also be requested at the check-in desk or gates, though sometimes agents are not familiar with the process.

Just ask them to search for REVUP in there systems - it worked great for me when taking advantage of the $275 fares to HNL a couple years ago.