Quantum Mistake

Quantum Mistake (originally titled Change Guy) is a manhwa written by Son Eun-ho and illustrated by Choi Myung-su. It was serialized from 1998 to 2006.

Plot
On the same day as a scientist is performing body teleportation experiments, Woo-Soo Choi and Kang Too-Jee end up in a fight after Woo-Soo Choi pulls down Kang Too-Jee's trousers accidentally. As they are fighting, the mad scientist - currently chasing a homeless person she was trying to bribe who stole the money - runs them over. She then uses them to test her new teleportation device. The transportation is successful and she then unceremoniously dumps their bodies and drives off only to find something was wrong with the device. She had just teleported their bodies, not their minds to go with them.

When he wakes up, Woo-Soo Choi sees himself lying on the ground. He realizes that his body has somehow been switched and runs his unconscious body to the hospital. When Kang Too-Jee - now in Woo-Soo Choi's body - wakes up, he appears to have amnesia and has reverted to a childlike state. Woo-Soo Choi's mother believes Woo-Soo Choi - in Kang Too-Jee's body - to be the one who caused it and chases him out of the hospital.

Woo-Soo Choi is able to find out where Kang Too-Jee lives and goes to school, but soon enough trouble appears in the form of Kang Too-Jee's old rivals and the fighting begins.