I've preordered a Pixhawk and my plan is to replace my APM 2.0 with the Pixhawk. I've looked around the 3DR site some and haven't really seen enough information on the Pixhawk to understand what I will really need regarding parts to do the upgrade.
I finally have some time to work with my tricopter project. I've been slowly getting things working. I just added the UBlox GPS to my APM 2.0. Nice upgrade - the tricopter no longer has the jitters in Loiter mode and the lock is super fast.