You can not avoid DNS issue. Your best time to switch a domain name is late at night and hope it resolves by the time you wake up it will take minimum 6-9 hours to propogate. In the old days it took up to 72 hours remember
Check the forum path your using in forum settings it better be your photopost url in there classifieds right. My guess is you have it set to classifieds url but your integrating with photopost right this is not a standalone right?