A courtesy upgrade from Y to J when Y is oversold and there are open J seats - it is suppose to be given out by Elite status and fare class, but agents don't often follow those guidelines.

It's a complete crapshoot and you can fly 50 times without ever seeing an OP-UP, so don't count on it at all.

If you want to upgrade to BF, you will need to pay for it with miles or cash+miles - there are no other options.