This excerpt taken from the PARS DEF 14A filed Oct 3, 2006.
Can I change my vote after I have mailed my signed proxy card?
Yes. You may change your vote at any time before your proxy is voted at the Meeting, as follows:
If you choose the first, second or third method above, you must submit your notice of revocation or your new proxy card to us or cast your votes on the website prior to the Meeting.