Hi Bigbri5050,
Unfortunately with PSR-S670/770/970 and older PSR-S models it is NOT possible to switch Registrations via received MIDI commands. This only works on PSR-S775/975 and Tyros2/3/4/5 (with Bank Select MSB/LSB + Program Change) as well as on PSR-SX600/700/900, PSR-A5000 and Genos (additionally with the new SysEx method that works both ways).
Best regards,
Chris