I downloaded the user manual a while back. The "new" model lacks the banks of programmable sliders (also used for drawbars). It lacks the MAXX sound processing. It lacks separate buttons for right hand sounds. Not sure if you can use the FC5 foot controller. Not sure if it has phantom power. (PA900 does not, PA3X does).
I'm sure I'm forgetting a few things. I can't see any reason to change from a PA3X, unless you have a 61 and want 76 keys.
Another important thing to me would be whether it offers a chord recognition mode that the PA3X had and the PA900 does not have. That is the mode that allows you, in split mode, using styles to play three note chord and not have the the chord change until you play another three note chord. This lets you play individual left hand notes between chords. I miss this. No way to really know without playing one.