I Thought It Was Rejuvenation

After trying to destroy the Jinsik that had been imprisoned for 40 years, he collapses and when he comes to, he finds himself in the body of a young man in his 20s.

"Hahahaha! I've become a master of Banrohwandong... Huh?"

He did get younger... but all the internal energy that he's trained his whole life for has disappeared?! I thought it was Banrohwandong, but it's possession! On top of that, the owner of the body is so frail that even his nickname is "MuMu (無武)". Kang Daesan, the sole successor of the Mugeuk School who lost his internal energy but gained youth, freedom, and a family. There's nothing he can do about it, martial arts skills just need to be recovered. The second son of the Changcheon Confucian School, the frail Yu Seok, begins his exhilarating journey!

(Source: Mr. Blue, translated)