First s*x scene, if you can call handjob that, happened at chapter 18. As expected of genre, reason sounds lousy, but it was hilarious how MC used secret technique to lengthen his d*ck because original was too small.

Because he doesn't have a personality to make long term plans, fight monsters or act proactively. Although most infuriating thing is how original's wife accepted changed behavior as Si-woo "not being young anymore", daughter is at most, mildly suspicious, but sister-in-law, who was overseas for 7 years and his student from guild found out that he is fake at once. Pitiful guy.

Lastly, he meets his wife as promising mage hunter and decides to seduce her. Not in a romantic way, but more of petty revenge for hiding secrets from original Si-woo (he only noticed it now), disregarding his opinion (understood it after taking a step back), just to see what face will she make after he reveals his identity. Her sister might had a crush on him by the way.

The protagonist, Son Siwoo, is a famous S-Class hunter and swordsman, known publicly as a hero, but in actuality his life isn't so rosy. His wife and daughter are cold to him, and he secretly suffers from an incurable internal injury that's slowly deteriorating his body and strength due to a past battle.

Then comes along Kim Dojin, a NEET loser who envies the life Siwoo displayed publicly. He happens to have a one-time use unique ability "Equivalent Exchange", which allows him to force an exchange that's perceived equal. He approaches Siwoo with the intention of swapping places with him, ignorant of his injury and frosty home life. Normally an exchange between him, a loser, and Siwoo, an S-Class hunter, would have been impossible, but due to the aforementioned injury, Siwoo's despair over his life, and envy of Dojin's youth, the exchange actually went through, and they wake up the next morning in each other's bodies.

At first, Siwoo is furious at having his body and identity unilaterally stolen from him, but quickly realises that Kim Dojin's body is incredibly talented at magic, something he as a sword user had never touched, despite his wife being one of the country's top mages. The unique experience of having his soul leave his body and enter a new one also granted him the unique ability to see mana, an advantage no other mage has. With the knowledge and experience he has as an S-Class hunter, he swiftly begins whipping his new body into shape to start a new life full of promise, while also dealing with past entanglements.