I just started running new ads that totally replaced my old ads. I think new ads are given a fair shake to see if they're improvements over the old ones. Mine were, most likely due to the fact that the search term happened to be the new web address I was promoting.
But other suggestions to turn of ad optimization probably will help your new ad to rack up a sizeable number of impressions, if so desired. I just wanted to show that it's possible that positive response for new ads can quickly displace old ads. :)