
I'm thinking this, too. This does happen in real life. It's likely a dissociative disorder, most likely depersonalization/derealization. I have DID, which includes those two symptoms, and it's uncanny AF. Him not recognizing himself in the mirror is dissociation, but then him behaving like Young Jihoon is extremely telling.
If the real Jihoon was in Wonjae, he would've likely tried to reach out to Youngmin, which would've been easy since he lives in the same place.
Also, Jihoon/Wonjae grew jealous of Youngmin with other people...
Of course, anything can happen. But yeah, dissociation makes the most sense right now.
Shin Wonjae never took over Guk Jihoon's body. Okay, hear me out. As we can see, Shin Wonjae is still alive. Which I had a feeling about even before he appeared. But that's not the point. I think Shin Wonjae and Guk Jihoon met before the accident happened since they all went to the same academy back then. There's a chance they had some brief interactions w each other in which Guk Jihoon got to know details about Shin Wonjae's personal life. And maybe, MAYBE after the accident trauma, Guk Jihoon started taking on Shin Wonjae's personality. And the body switch never happened actually. Which will also explain why Shin Wonjae claiming to be in Guk Jihoon's body seems to do/say things the "real" Guk Jihoon would do/say according to Noh Youngmin's reaction before. And that will also answer my question as to why the Guk Jihoon in Shin Wonjae's body never tried to contact his family and Noh Youngmin to inform of his survival?