What happens if a petition is filed by a Legal Permanent Resident (F2A or F2B) and the petitioner becomes a U.S. citizen before the applicant is called for an interview?

When the petitioner naturalizes, the F2A petition (spouse or minor child of a permanent resident) is automatically converted to an IR1 (spouse of an American) or IR2 (child of an American) petition. These visa categories are not subject to the visa quota system and will therefore always have visas available for the beneficiaries’ use if they qualify for visa issuance.

However, because there are no derivative beneficiaries allowed with IR petitions, children who are derivative applicants on a parent’s F2A application must each have a separate IR2 petition filed for them if the petitioner becomes a U.S. citizen. F2A petitioners who naturalize should make sure to file new IR2 petitions for their children (if they did not originally file separate petitions for them).

For F2B applicants (unmarried son/daughter of a permanent resident), their F2B petitions are automatically converted to the F1 category retaining the original priority date when their petitioners naturalize. Because of unique circumstances, the waiting period for the F1 category is longer than the F2B category in the Philippines. However, under Section 6 of the Child Status Protection Act (CSPA), the applicant (not the petitioner) can request exemption from the automatic conversion of the visa category from F2B to F1 by submitting to the Department of Homeland Security/U.S. Citizenship and Immigration Services (DHS/USCIS) a written statement that he/she elects to have such conversion revoked. The applicant may send the written request for retention of his/her F2B category to the DHS/USCIS office in Manila by fax at 301-2208 (Attn: Field Office Director) or by mail: DHS/USCIS, U.S. Embassy, 1201 Roxas Blvd., Ermita, Metro Manila 0930.

Source: http://manila.usembassy.gov/wwwh3201.html