Korean will often op-up to a business seat with economy service if economy is full. I was upgraded both GMP-KIX and ICN-MNL. GMP-KIX I had no status, but ICN-MNL I was Delta Silver (Sky Miles Elite). They also upgraded my two friends traveling with me (separate reservation, no status, just check in together).
It's best to ask at check in if the flight is over sold. If they say yes, ask them if you can be moved. It's a rule they all know and practice often, not like a big secret that it's rude to ask.