I'm trying to figure out if there are any tricks to booking an upgrade with CO miles on Star Alliance partners. I'm planning to go from LAX to BKK, then MEL and back to LAX in March/April and it would be great to grab some upgrades. From what I understand you have to purchase a Y or B fare, and then ask for the upgrade. But that just seems crazy. Why would I pay a premium for the fare without some indication of whether there is an upgrade available? I have plenty of flexibility on dates, and I would pick the airline and flight based on whether I could get the upgrade.

I'm not sure if I'm missing something or if this is actually the process, who would take a chance like this?