It was a software-only update, which was much appreciated because unusually, it actually added something useful rather that just the usual "fixed vario0s bugs".
The main things it added were the chord looper (brand new feature) and it "unlocked" more expansion memory taking it to 3GB total.
It also included a free YEM pack called "Genos Superior Pack" which bundled together a bunch of stuff from some existing premium packs and some voices that were new on the SX900. This was before all the YEM packs were free.