Yahoo! announced that the adCenter transition will begin on Oct'1th. "Starting Monday, you may start seeing less impressions, clicks and costs reported in Yahoo Search Marketing console. If you've set up an adCenter account, then you likely will see an increase in impressions, clicks and costs on adCenter side."
"YSM advertisers may see traffic briefly decrease in their accounts during these testing periods. When ad serving begins to transition to adCenter, you're likely to experience a decrease in clicks and impressions in your YSM account. And, at the same time, it's a best practice to monitor your adCenter budgets closely during testing and be ready to increase your spend if your budget falls within 20% of being fully spent. When ad serving begins to transition to adCenter, adCenter account clicks and impressions are likely to increase," explained Yahoo!.