We present numerical models of mantle convection incorporating changes in viscosity and thermal conductivity at depth. Increasing the viscosity and/or thermal conductivity by a factor of 5 at depths below 2000 km increases the size and stability of thermal upwellings. Decreasing the thermal conductivity beneath 2000 km, in contrast, has comparatively little effect on thermal upwellings. Increases in viscosity and/or thermal conductivity close to a factor of 5 beneath 2000 km may tend to counteract the tendency of the post-perovskite phase transition to decrease the size and stability of thermal upwellings. We also find that varying viscosity and thermal conductivity beneath 2000 km affects basal heat fluxes and mantle geotherms, which should have strong implications for the stability of the post-perovskite phase change near the core-mantle boundary (CMB).