Changes to AdSense on Google Sites
Starting October 10, 2013, it will no longer be possible to modify AdSense ad code on Google Sites pages. However, any ads you’ve previously placed on Google Sites will continue to show, and you’ll be credited for all valid earnings.
No change is required on the part of AdSense users. Any ads that you currently have running will not be altered or removed, though you’ll be free to delete ads yourself. However, it will no longer be possible to edit or add new AdSense ads to your existing Google Sites site or to new pages.
Please be assured that this update will not affect the standing of your AdSense account or ability to show ads on any other websites you own. If you’d like to show ads on other websites, simply log in to your AdSense account at www.google.com/adsense to generate new ad code at any time.