Hi Mike,
You don't necessarily have to change the Style (as suggested by Gary). You can also save any changes you make in the Mixer (both to the Keyboard voices and the Style voices) directly to Registrations (as mentioned by ckobu).
So go to the "Style" tab in the Mixer, change the voice for the desired Style part (channel), adjust the volume and re-memorize the current registration by pressing Memory and then pressing the current registration button again. If necessary, repeat this for further registrations of this bank. Finally, the registration bank must be saved again. You should not change the file name, otherwise this bank would no longer be found by the existing playlist.
Important: After pressing Memory, please make sure that the necessary boxes are ticked or that no boxes are ticked for parameters that you do NOT want to memorize in the registration (e.g. pedal or assignable buttons). The following boxes should at least be ticked for Style registrations: Voice, Tempo, Style, Multi Pad and Keyboard Harmony/Arpeggio (!).
Hope this helps!
Best regards,
Chris