The actual significance of this change
Despite what some people might think, i believe this does in fact confirm the old jitae is gone. The entire scene was leading up to him becoming free and what best way to become truly free then to liberate yourself from the suffering that is "you." I believe jitae during the tunnel arc is the same jitae during the beginning of the series. While seemingly entirely different, he is at his core the same person just incredibly broken. We see this in the fight with sunny jaa where for a brief second he returns to being afraid and vulnerable. In essence, its his whole personality is cope. Not a "real" change. In truth, the only actual moment he truly changed as a person and adopted a new personality was in this metapohrical moment of "killing" his past self. Now that his old self is gone he has no burden set from his previous self. I believe that his view on violence is different as well now. Even though before during the tunnel arc he still hated violence, he exacted it and justified it under this drastic persona. Now that he has actually "changed" it won't be just a coping mechanism but who he actually is now since he won't be suffering as he is doing it. Basically now his actions do not contradict his beliefs so its not coping mechanism therefore he isn't broken. A lot of people assume he is just gonna be a crazy crashout the rest of the series which i believe is far from the truth.
I also want to add that the combat buff he got wasn't from him gaining power necessarily but from his mind being one with his actions. Before while he was improving at a rapid pace, he was held back from his fullest potential due to his own morals being crushed into the dirt from his actions. Think about it like this, would you truly be the best you possibly could be at something if you have very little or no motivation at all? If i were to pursue a career i do not enjoy I probably will not get very far at it because its a conflict of interest. That is the point here. Jiu ji tae up until his metaphorical self murder did not like violence. He was a walking contradiction which likely stunted his fighting capability. After coming to terms and liberating himself he tower away from any shackles that held him back in battle. You're mind is the most powerful weapon and the your greatest weakness depending on its usage.