According to USCIS as released in the Vermont Service Center Stakeholder Newsletter issued on or about July 19, 2012, Form I-130 filed by a lawful permanent resident who later becomes a naturalized U.S. citizen will now automatically become upgraded once the petitioner becomes a naturalized U.S. citizen.
In the past, when a lawful permanent resident becomes a naturalized U.S. citizen, the petitioner would need to send USCIS a letter and a copy of their certificate to reflect their new status as a U.S. citizen. However, now the Vermont Service Center has upgraded their system by creating the program that tracks the petitioners’ alien numbers and will automatically upgrade their status in the system once they become a U.S. citizen.
The difference is that a U.S. citizen does not have a waiting period when sponsoring their spouse or parent. A lawful permanent resident, however, does have a priority date when sponsoring their spouse and they will need to wait until their priority date becomes current before their application will be sent to the National Visa Center for processing to prepare the paperwork for the U.S. Embassy.
If however the Form I-130 has Already Been Approved, then you must notify USCIS and / or the National Visa Center of the upgrade in your status by providing a copy of the naturalization certificate.