So what can be interchanged?

[guruerror]

Exterior...from what I can tell, almost everything. One thing to note, the front grills can be swapped, but they must be done as a set (upper and lower together). The lower grill from an 03-07 will not fit on the upper grill from an 08-09 and vice versa. The clip slots do not line up.

Interior...almost nothing related to the dash, gauges, trim, unless you are doing a complete swap, but then you might even run into wiring issues with harnesses not being long enough, plugs being different, etc. The newer H2s might even use the CAN bus...I'm not sure about that though.