I think it starts right at the Budokai tenkaichi. At the end of the fight, Goku gives Piccolo a senzu, and claims him to be a “rival”, rather than the Demon king who killed his friend. read more
When he is revived, he has had a bit more time to settle into his personality, and this basically becomes the Piccolo of the rest of the series. During Freeza he's indifferent to Goku, then befriends him properly when training for the Androids. read more