According to a USCIS Press Release: “U.S. Citizenship and Immigrations Services (USCIS) announces publication of a Federal Register Notice that expands the Direct Mail program to include Form N-400, Application for Naturalization. The notice advises the public to file non-military N-400 applications with the appropriate Lockbox and provides a 30-day transition period whereby the Service Centers will forward any N-400 applications received at their facilities to the appropriate Lockbox. Applicants should continue to file Military N-400 cases at the Nebraska Service Center (NSC). In addition, in order to assist military spouses, NSC will also accept and process N-400s filed by spouses of active members of the military.”
The USCIS has also released a related FAQ on this development.