He didn't change his personality, the rod did by damaging his brain.
Also, I don't believe personality changes with age. Personality traits become more pronounced. I think anyone that has ever known anyone from adulthood to elderly (or who works in a retirement home/village) knows and believes this.
So if I stuck a metal rod in my head would
1.) the rod be sticking itself in my head
or
2.) me sticking the rod in my head