Density functional calculations are carried out to study the symmetry and substitution-driven electronic phase transition in BaPb1−xSnxO3. Two end members, BaSnO3 and BaPbO3, are found to be insulating and metallic, respectively. In the latter case, the metallicity arises with the presence of an electron pocket, formed by Pb-s dominated conduction band edge, and a hole pocket formed by O-p dominated valence bands. While electron carriers are found to be highly mobile, the hole carriers are localized. Our study reveals that an insulating phase can be realized in the metallic cubic BaPbO3 in three ways in order to explore optoelectronic properties: first, by lowering the symmetry of the lattice to monoclinic through rotation and tilting of the PbO6 octahedra; second, by hydrostatic pressure; and third, by alloying with Sn substitution. The presence of soft phonon modes implies the plausibility of symmetry lowering structural transitions. Furthermore, unlike the earlier reports, we find that Sn substituted BaPbO3 cannot exhibit the topological insulator phase due to the absence of the band inversion.