I am trying this now in 2025. It was a struggle to work it out. You can no longer just go to a Global Entry Enrollment Center in person without an appointment supposedly. I live in Boston Area. Logan Airport is by appointment only and is booked out 2.5 months at least. I almost drove to Vermont again (as I did for my original Global Entry) but not sure I can just turn up these days. Pretty sure when I did it before the pandemic they said I could. You cannot make an appointment without a receipt number (when you call Homeland) or an approval on you dashboard with a "make an appointment" option, you cannot trigger that unless you apply it seems. So its not possible.
I do not need to re-apply, there is still 4 years on my Global Entry card, I just need to make a change/update after citizenship change.
You can access https://ttp.cbp.dhs.gov/schedulerui/schedule-interview/location?lang=en&vo=true&returnUrl=ttp-external&service=UP and if you can see availability but you cannot book without being logged into the site. To log into the site you have to leave and go to https://ttp.dhs.gov/ which is deemed, "leaving" the scheduler site so you cannot get the login to register with the scheduler to allow you to book an appointment.
I have since read you can go here https://www.help.cbp.gov/s/questions?language=en_US and Choose
Topic - "Trusted Traveler Program"
Applicable Issue - Update document (at the bottom)
TCC Program - Global Entry
email - I would suggest using the same email you used for your global entry contact
Description - say that you are trying to update your Global Entry information as you applied with your past information (Green Card and Passport) and now you are a US citizen.
Upload a scan of the main id Page of the new US passport.
On the next page add you passport info from the new US passport
Then click Submit.
I will see how long it takes to get any feedback. Hopefully its this simple. If I have to go to an enrollment center I hope at least this way they correct the issue and enable me to make an appointment online from the dashboard again.