Subwavelength nonlinear optical sources with high efficiency have received extensive attention, although strong dynamic controllability of these sources is still elusive. Germanium antimony telluride (GST) as a well-established phase-change chalcogenide is a promising candidate for the reconfiguration of subwavelength nanostructures due to the strong non-volatile change of the index of refraction between its amorphous and crystalline states. Here, we numerically demonstrate an electromagnetically-induced-transparency-based silicon metasurface actively controlled with a quarter-wave asymmetric Fabry-Perot cavity incorporating GST to modulate the relative phase of incident and reflected pump beams. We demonstrate a giant third-harmonic generation (THG) switch with a modulation depth as high as ∼70dB at the resonant band. We also demonstrate the possibility of multi-level THG amplitude modulation for the fundamental C-band by controlling the crystallization fraction of GST at multiple intermediate states. This study shows the high potential of GST-based fast dynamic nonlinear photonic switches for real-world applications ranging from communications to optical computing.