You can change the adTagURL after the movie starts. However it will apply only for the next movie (when using a playlist) as the adTag configuration is read only once per video (when it first loads). The following code demonstrates how to change the adTagUrl when changing media (for example when you click on another video on a playlist). The code is added in the embed code readyCallback block:
'readyCallback': function(playerId){
var kdp = document.getElementById(playerId);
kdp.kBind("onChangeMedia", function(){
kdp.setKDPAttribute("doubleClick","adTagUrl","http://pubads.g.doubleclick.net/gampad/ads?sz=640x360&iu=/6062/iab_vast_samples/skippable&ciu_szs=300x250,728x90&impl=s&gdfp_req=1&env=vp&output=xml_vast2&unviewed_position_start=1&url=[referrer_url]&correlator=[timestamp]");
});
},