We use DoubleClick for Publishers (DFP) Small Business.
Can I change the AdSense account that was used when we created the Doubleclick account? I'd like to link a different AdSense account.
It appears that your AdSense account is permanently linked to your DFP account.
See this discussion on the Google product help forums: Created new AdSense account, but old one linked to my DFP account. Other people have had the same issue and there are some tips on migrating advertisements from one DFP account to another.