To meet the increasing demand for environment-friendly, high-performance energy devices, sodium niobate (\ce{NaNbO3}) is considered one of the most promising lead-free antiferroelectric (AFE) oxide perovskites for such green energy storage applications. However, as disclosed by recent experimental reports, under an external electric field, the room-temperature AFE $P$ phase of \ce{NaNbO3} has been demonstrated to undergo an \textit{irreversible} phase transition to the ferroelectric (FE) $Q$ phase. This puzzle challenges our current atomic-scale understanding of this field-induced AFE-to-FE transition, and thus hinders the widespread use of \ce{NaNbO3} in lead-free AFE energy storage devices. To unravel this puzzle, we perform first-principles density-functional theory calculations to establish phase stability maps of the \ce{NaNbO3} polymorphs determined from group-subgroup relations. For the first time, we identify two new key intermediates ($P^\prime$ and $Q^\prime$) via the symmetry-adapted phonon mode analysis based on high-symmetry cubic phase and minimum energy pathway transition state searches, that facilitate \textit{de novo} phase transition pathways for the switching of polarization with significantly lowered energy barriers. By means of a phenomenological Landau-Devonshire model, we predict and explain why these new intermediates can rationalize the persistent lack of a double polarization-electric field hysteresis for \ce{NaNbO3} under an applied field. This sets the design platform for future precise engineering of \ce{NaNbO3} at the atomic-scale for lead-free AFE energy storage applications.