@Luis C. Why you r thinking that way....he may have reincarnated and that was his body from the beginning....he just remembered the past just at that moment.... And as per the name of the manhwa I think it's the most plausible theory
I guess you could say if it was a 'possession' it's stealing a person's body and if it's 'reincarnation' with past memories resurfacing it's just part of the natural process but his memories resurfaced
(manhwas use them interchangably in titles so we can only know the later chapters)