You can use enter and exit the BB menu eight times to reset the Tournament prizes (as long as you're not involved in one at the time) and use Reset Data to change the League prizes.
Tidus (0:0:0 on chocobo race)
Wakka (Endless Blitzballing)
Lulu (200 lightning bolts)
Yuna (defeat all of Belgemene's aeons)
Auron (all fiends in 10 areas)
Kimahri (Butterfly catching)
Rikku (Cactaur village)
You can use enter and exit the BB menu eight times to reset the Tournament prizes (as long as you're not involved in one at the time) and use Reset Data to change the League prizes.