Tom, I have played around with the PSR-e403 quite a bit. You cannot "revoice" the factory styles, but you can load one user style into RAM. You can have 5 user styles in the board at any one time, but only one available for use.

The style has to be pretty simple (I think the .sty file has to be less than 36KB). I figured this out thru trial and error. Obviously you have only two variations, intros, endings and fills. The tracks can contain standard Midi voices or "XG Lite" voices. I successfully created a user style with a different bass voice.

You might be able to edit a commercial style with some of the many 3rd party utilities, that are available for Yamaha styles.
Good luck!
Ted