Wirelessly posted (Samsung Omnia: Mozilla/4.0 (compatible; MSIE 4.01; Windows CE; PPC; 240x320))
As far as I know, the origin and destination need to be the same. The routing can change though. They will most likely charge you a change fee and any difference in fare to make the airport change you want. You can hope for some bad weather or some type of delay and ask to be re-routed.