Note: my research could only confirm the setup, not the full plot, so this analysis is cautious about specifics.
The swap is the engine. Pan-soo, living as Dong-hyun, learns that Hyun-jeong is his daughter and that Mi-seon was left to raise her alone. The structure moves him from self-interest to protectiveness: the bully-beating comedy and the gangster-world conflict converge on his decision to put his daughter before his old life. The emotional payoff is the reunion between father and daughter and the reckoning with Mi-seon, which at least one viewer found awkward. The hidden meaning is that the body swap is less about trading bodies than about giving a man the humility he never had, and Dong-hyun's own life is a mirror for what the gangster has missed.