That would probably depend on whether they have extra meals or not. So I don't know if it can predicted with 100% accuracy.

And even if you can, you might not have your choice of regular meals by that point (ie, only one of the regular meals might be available as an extra, even if there was a choice of regular meals originally announced).

They also might make you wait until everyone else has chosen their meal before they would let you choose a different one, since why should you who ordered a special meal get priority over anyone else for a regular meal?

Why would you change your mind only after you get on the flight? I'm not familiar with Korean Air specifically, but with most airlines you can make or change the special meal request up to 24 hours (maybe a bit more in some cases?) before the flight.