I'm assuming you're on OZ204 (the late flight) and went from an A380 to an A350.

You have a couple of options depending on when you want to leave ICN, A) is to ask OZ(or maybe AV) to move you to OZ202 (the early flight) if there's still F available, or B) have them refund you the mileage difference between F and J. I haven't experienced full OZ F (just the F seating upgrade w/ J service to NRT) yet, but J on A350 is still an excellent all around product so you're not dealing with a major downgrade, just make sure that you get your mileage difference back.

It's usually the late flight OZ204 that gets the smaller plane when they need to swap things around (380 to 747, 777 or 350).