AFAIK, the bumper should be pretty easy to swap. Was pretty easy to take mine off (99.5), even when I didn't use a Honda Civic to do it. hehehe.. whoops.
My guess is the grille will not fit, at least not without a lot of duct tape or crazy glue. LOL. That said, I do not know how the front of the 96-99's come off... the 99.5+ has a center piece. This should give you an idea:
http://www.nissanpathfinders.net/forum/index.php?showtopic=26517&st=0&p=488677entry488677
As for headlights, my opinion is leave em as they are. But that's my opinion... I know plenty of people who like to upgrade to burn your eyes out bright. Personally, I can't stand passing people who have done that.