Pete- Unfortunately I cannot try this myself or give you detailed instructions as I do not have, and never did have, this keyboard. But be sure that you are opening up the style channels on the mixer. The top level of the mixer shows accompaniment as a single entity that can be adjusted. But you can usually also open up all the style channels on the mixer. You should then see the setting of each part, including the voice. When you select that choice it lets you change it.
Maybe this just doesn't work on the 8000, but it would surprise me. We need someone else with an older keyboard, or who remembers the 8000 and similar PSRs to chip in here.
Mike