The Nissan handsfree doesn't work but I never had that to start with. The f900bt has built in bluetooth which works perfectly with the correct adapters etc as do all the other steering wheel controls.
You do not have to change everything at all, I didn't to start with but when someone on here was selling some lovely speakers I couldn't resist. The Pioneer unit sounds a LOT better than the originial Bose, but you do need some boosters otherwise the volume level is pretty low
Have a search in the I.C.E. section there is a great post by a guy called srobrien and also I think BulletMagnet has posted some info too as he has a f900bt.
Whatever you do don't pay £150 for the 3.x update card from pioneer, I can talk you through doing it for free other members can confirm this It makes the unit so much quicker and easier to use than the old 2.x software
Any questions just drop me a PM, happy to help