take your current pet to a tamer. not sure where they all are, 1 is outside ascalon city, another outside breaker hollow (archipelagos exit) i think. the tamer will take your pet, then just have charm animal equiped when you go to fight shiro, after you kill him you will get sent to devine path where there is the phoenix to be tamed (should be at least 2 there)
can i have your 100g? =D lol jk, goodluck getting the pheonix, but just so you understand, pheonix are level 5 and youll have to relevel them. so no you can't "switch" but you can easily tame a completely new pheonix.