Originally Posted by gpandaleon

The FAQ suggesting that you can change your application and pay the difference in the fee is wrong. I finally connected with CBP through their email help facility, and they were reasonably responsive (4-6 hours for a reply). You cannot modify or even cancel your application once it has been submitted and the fee is paid. They have to do this for you, and the fee is forfeited. Once they cancelled my Nexus application I was able to apply to renew my Global Entry membership.

Nexus provides everything that Global Entry does, costs half as much ($50 vs $100) for the application and also provides fast passage across the Canadian border in both directions. That's the good news. The bad news is that it takes 3-4 months to process the application, and an interview is required. The interview locations are limited to Canadian border crossings, so if you don't live near the border, and don't travel frquentlty to Canada on the ground it is a nuisance to get to the interview. I live in Chicago, so the closest border crossing for me is in Detroit.