Before starting the game, Player-A has a Basic Pokémon but Player-B does not. On Player-A's draw 1 card for mulligan, say they draw a better choice for starting Pokémon; are they allowed to switch it out with the one they chose to be their 1st active before the mulligan? And if they choose to switch, must it stay on the field and not allowed to be put back into hand?
Player A continues his or her setup and can choose to draw an extra card after setting aside his or her Prize cards. Once placed, the active cannot be changed during setup, but a mulligan card can be placed onto the bench.
Source: PUI Rules Team (2005-05-05), PUI Rules Team (2006-02-09)