This excerpt taken from the PBG DEF 14A filed Apr 4, 2007.
Can I change or revoke my vote after I return my proxy card or voting instruction card?
Yes. Even if you sign the proxy card or voting instruction card in the form accompanying this proxy statement, you retain the power to revoke your proxy or change your vote. You can revoke your proxy or change your vote at any time before it is exercised by giving written notice to the Secretary of PBG, specifying such revocation. You may change your vote by timely delivery of a valid, later-dated proxy or by voting by ballot at the Annual Meeting. However, please note that if you would like to vote at the Annual Meeting and you are not the shareholder of record, you must request, complete and deliver a proxy from your broker, bank or nominee.