It's a parallell world thing. From the time he receives the letter from the co-worker, the world branched into two, one where he chose the shorthaired childhood friend, and another where he chose the OL. The protagonist(s) discovers they have the ability to enter each other's worlds if they wish for it before opening their apartment door, and decides to trade places... That's where the title, "Koukan" comes from, exchange.
Base +9, Sky boy +6, DOblH +8, AragonOfWorld +6, dhaljarraque +26, Ejastik +7, Dune69 +14, M19M1rr0r +9