Previously, when an advertiser made changes to his ad, the changes will overwrite the existing ad copy, but he still retains the same ad id for that particular ad. If the advertiser makes changes to his ad now, his modified ad will over write the existing ad, but more importantly he also gets a new ad id and loses his old ad id. You may ask so what difference does it make?

Example: An advertiser creates 2 new ads for his ad group chocolates. Yahoo would assign unique ad id to both ads 1 & 2 created by advertiser. Advertiser checks the ad performance and finds that ad1 performs better than ad2. He changes the ad which didn't do well and lets assume its ad2.

Advertiser has to be careful, if he is making changes to an existing ad, which has historically performed well and has a good quality index. If he makes changes, it's not only going to over write an existing ad but also delete an ad which has good quality index. The advertiser should pause the ad and create a new ad. So even if the new ad doesn't perform well or has poor quality index, he can switch the paused ad2 which at least performed better.

Advertisers can check the new update by looking at the ad performance report. It clearly shows performance data for both the old and the new ad with different ad ids.