I would add some info based on something that happened to me recently.

I wanted to upgrade from Economy on a flight from HK to Seoul. I had already ticketed my flights in K class which is upgradeable.

However, my outbound flight had no PE, so I could only to economy to biz; while the return flight had PE so I could only do economy to PE ... which meant I COULD NOT do this online for some reason, and when i tried ringing up, could not get the "return" rate for the upgrade just the much more expensive 2 x one-way upgrade. Really irritating.