Hi Rikki,
Like you, I thought I would update my PR900 piano but after having played the 804 for nearly two hours, I am going to stick with it as in my opinion it is a much better piano. The new ones have what I can only describe as a 'dead' spot on the keys resulting in keyclick which I hate. Apart from a few up-do-date rhythms and sounds (which I already have on my lovely KN7000 keyboard) for me it really isn't worth the change.
With regards to using the piano mode, to get the rhythms anywhere on the piano, I press and hold the SPLIT button until the keyboard appears in the home page, then press the highest note in the treble, release and wait until the keyboard disappears from screen. You can then play anywhere on the piano using your own chords and rhythms. Works for me anyway. Hope this helps.
Aud