[Guest Post from Immigration Lawyer Ms. Teodora Purcell; with no tax discussion.]
A US diplomatic or consular officer is authorized to certify the facts on which you may have lost citizenship by completing a Certificate of Loss of Nationality (“CLN”), and some other forms.  The CLN must then be approved by a division Chief in the Office of American Citizen Services and Crisis Management (CA/OCS/ACS) in the Directorate of Overseas Citizens Services, Bureau of Consular Affairs of the DOS, or the Acting Division Chief.
If you have a change of heart after appearing at the US Consulate and signing an Oath of Renunciation or the other related DS forms but before the CLN is approved, the Consular officer will notify the DOS and ask them to deem the request for loss of US Nationality withdrawn.
If the CLN has already been approved, you will have to seek administrative review of the loss of nationality finding through the DOS Office of Legal Affairs, Bureau of Consular Affairs.
When the DOS issues the CLN, it will cancel your US passport or naturalization certificate.
 7 FAM § 1213b
 7 FAM § 1228.2
 7 FAM § 1230
Teodora Purcell | Attorney at Law
11238 El Camino Real, Suite 100, San Diego, CA 92130, USA
Direct: +1 (858) 793-1600 ext. 52424 | Fax: +1 (858) 793-1600