The ability to tune the Fermi level of semiconductors is at the heart of modern electronics. Here, we demonstrate that persistent photoconductivity (PPC) enables tuning of carrier density, conductivity type, and, consequently, the valley polarization in (Pb,Sn)Se/(Pb,Eu)Se quantum wells. Illumination of these samples induces Fermi level shifts that convert the system from a threefold-degenerate $\bar{M}$-valley two-dimensional hole gas to a single $\bar{\Gamma }$-valley-polarized electron gas with similar values of mobility. The optically induced state persists for more than ${10}^3$ minutes at cryogenic temperatures and enables stepwise optical gating without the need for device processing. These transitions are confirmed by the sign inversion of the Hall resistance and the modification of quantum Hall plateau degeneracies measured in magnetic fields up to 35~T. $k\cdot p$ calculations of the Landau-level spectrum quantitatively reproduce the experimental data. Furthermore, studies of weak-field magnetoresistance demonstrate the significance of quantum localization phenomena at the transition between the weakly and strongly localized regimes in compensated narrow-gap semiconductors. Spectral studies allow us to identify the critical role of the barrier material and determine the photon energies that can reverse the PPC effect. The persistent light-induced upward shift of the Fermi level in the $p$-type quantum well is explained in terms of specific energy positions of donor and acceptor defect states in the studied system. Our results demonstrate that PPC is a powerful optical gating tool for the IV-VI quantum wells, a versatile platform for reconfigurable valleytronic architectures. Furthermore, we discuss possible mechanisms allowing for fine, persistent, and reversible tuning by illumination between hole and electron conductivity and the relevance of our results for realizing the quantum spin Hall and quantum anomalous Hall effects in topological crystalline insulators.