No problems whatsoever in changing your FFP number when checking in. I do it all the time, for exactly the same reason as you. I give them my OZ number, do my advance seating assignment online, then change the number at check-in. Just be sure you double check the boarding pass to be sure they changed the number. They're very good at ICN, and I don't think they've ever missed it there, but at outstations they're very hit-and-miss about correctly changing the number, sometimes I have to repeat it 3 times (and printing out the boarding pass 3 times) before they get it right. If for any reason you have an OZ flight into ICN, then connecting to another OZ flight, and want to credit to OZ on one and a different FFP on the other (or credit to any differing FFPs, not necessarily OZ), you will really run the risk of them misunderstanding. I checked and doubled checked in such a case, and even had the correct number on the boarding pass, but it ended up in the wrong FFP because the agent at check-in changed it after she printed my boarding pass. I should have checked again when I arrived at ICN. But if you're just talking about a single flight, or just crediting to one FFP, it should be fine if it shows correctly on your boarding pass.