Google is retiring the old version of AdSense for Search and telling users to switch to the new version powered by Google Custom Search, which has been offered for a while.
If you see "powered by Google" on the logo on the search results page, then you're using the old one. In a post on Google's Inside AdSense blog, Katrina Kurnit writes:
AdSense for search with CSE gives you more control over your search results without changing how you earn money showing AdSense for search ads. For example, with the new version, you'll have access to advanced features like refinements and promotions. Our team has developed a number of updates and improvements during the past few months, and you can access more advanced features at www.google.com/cse.
Your existing AdSense for search box will continue to work normally for a few more months, and we'll be sure to update you when we retire this version. To take advantage of the benefits offered by the new version of AdSense for search and ensure you don't miss any revenue during the transition, we encourage you to update your code now.
To upgrade go to AdSense setup and "AdSense for Search" then select the sites you want your users to be able to search across, customize the look and feel and update the code for your site. Google offers more info here.