Never got one but was told in NRT before at check-in that I received an opup from Y to PEY and might get one later to J. Unfortunately didnt happen in the end.

I guess you have to be extremely lucky and a couple of factors have to happen.
- Heavily overbooked.
- No other members in the next cabin class, probably not even GR.
- No higher tier members who also got upgraded to the next class