I noticed the biggest difference is that the older 03, 04 seats manually reclines where's the newer 05+ is powered.

 

Just FYI for every 03 04 g35c owner. So I installed 05 seats in my car and everything fit perfectly. The problems that I have come across are the following things:

1) Both driver and passenger Seat doesn't slide forward by itself when flipped forward to allow rear occupants out.
2) The little button on the back of the seat to have to seat slide forward but on the driver side it slides only about 4 inches at a time forward, and the passenger sides doesn't even work at all.
3) Both seats doesn't slide back into it's original position after being flipped back from letting rear occupants out.

Other than these little problems which I plan on modifying to make work the seats are great. All of the power switches work (except passenger side rear button) and heated seats work also.

hope this helps!