High-pressure, high temperature reaction conditions can be useful to stabilize Pb2FeTeO6.5 metastable polymorphs of complex transition metal oxides. We successfully prepare a new defect pyrochlore Pb2FeTeO6.5 with B-site disordered Fe and Te cations under ambient conditions. Treatment of this material under 8 GPa and 950 degrees C results in a reductive transformation into the B-site cation-ordered double perovskite Pb2FeTeO6. MOssbauer and EELS spectroscopy confirm the iron cations are in the +3 oxidation state in both phases indicating that this transformation proceeds via reduction of the tellurium cations under apparently oxidizing conditions. This reaction demonstrates that for a suitably chosen system, it is possible to carry out chemical reactions under pressure in unexpected ways.