A ton of work went into this thing, mostly from moon_wizard (all the effects, combat, etc) and Zeph (the new look).
Also my poor testing groups had to struggle through a few sessions where things just didn't work (locked tokens = map crash. insubstantial = 1.5 damage instead of 2.0 damage, etc).
To check it out you'll need the TestModeManager tool for FGII which can be found here
After that you should just update and it will download all the new stuff. I strongly recommend backing up your campaigns before doing any conversion, but I don't remember any upgrade issues (then again 1.5.1 was months ago for me and my testers).
You can find more details on the thread in the FGII forums. I recommend you clear time on your schedule to read the changes/readme files once the install completes. A lot of things have changed...for the better.