24 hours before your flight to NRT I would go to Barajas Airport and go the the Iberia "Business" Sales Counter. this counter is a stand alone counter right by the Business Class checkin counters. I would approach them and ask them you want to upgrade to Business. they will tell you if you are eligible or not. it will be a flat. i paid 499 Euro from MAD to ORD. The flight to Japan may be longer and they may ask for more money. Good luck