Endospore formation in Firmicutes represents a complex survival mechanism in response to environmental stress. While the process is well studied in monoderm bacteria like Bacillus subtilis, less is understood about diderm spore-formers such as Acetonema longum, which retain and remodel their outer spore membrane (OsM) into a true outer membrane (OM) during germination. Using a combination of structural and biochemical approaches, we aimed to investigate the molecular mechanisms driving membrane remodeling. Our results revealed that outer membrane proteins (OMPs) are present in vegetative and germinating cells but absent in mature spores. Notably, the essential β-barrel Assembly Machinery (BAM) component, BamA, was also missing from mature spores, suggesting that an inner-to-outer membrane remodeling occurs during germination. Expression profiling across growth stages identified two previously uncharacterized proteins, SonA and SonB, co-expressed with BamA and part of a conserved operon among diderm Firmicutes. SonA is a β-barrel OMP with three POTRA domains, while SonB is a predicted OM lipoprotein. In vitro, BamA and SonA spontaneously fold and insert into liposomes, supporting a model in which BamA and/or SonA initiate self-insertion into the OsM. We propose a model in which BamA, facilitated by SonA and SonB, drives inner-to-outer membrane conversion during germination in A. longum. This process includes the insertion of other integral OMPs, such as LptD, which further modify the protein and lipid composition of the OsM into a true OM. Collectively, our findings provide new insights into the functional properties of BamA and its crucial role in membrane remodeling and OM biogenesis.