The initial plan called for Mike Piazza to switch positions, switch leagues, switch coasts. It was supposed to be an opportunity for him to reinvent himself in the twilight of his almost certain Hall of Fame career, a chance to rest his beat-up 37-year-old body and do full time what he does best: hit a baseball a long way. Instead, Piazza decided on nearly the opposite, and he did so without an apparent trace of regret. He signed a one-year, $2 million contract with the San Diego Padres, who expect him to be batting cleanup and starting at catcher on opening day. At his introductory news conference yesterday in San Diego, Piazza said he was looking forward to catching as often as he is asked. He even acknowledged that the prospect helped lure him away from joining his hometown Philadelphia Phillies.