John Cena says his recent hair transplant has "completely" altered his life ... and he's ticked he didn't get the procedure done sooner.
The WWE Superstar -- who revealed back in April he had the scalp surgery after fans taunted him over his thinning hair -- went in-depth on the matter with PEOPLE this week ... and it ain't hard to tell he's loving his new look.

"It completely changed the course of my life," he said.