.sfc was an extension used for projects before .sfm became the standard. There should be no problem leaving the extensions as .sfc.
Paratext sets the extension for a project under the Typical File Name of the Advanced tab in the Project Properties. Once a project has been created, you cannot change the extension type from within Paratext.
You can create a new project with the extension set to .sfm and then import the books into the project. In the new project the extensions should now be .sfm.