On the fleche I did, two of the guys carried fresh shorts/tights with them. I've never been so jealous in all my life.
I've done a number of 24-hour events - some with the opportunity to change, some without. While it's amazingly nice, it's also okay to do it in one pair, as long as you've tested them and know the chamois is up for the challenge. Do at least a century in a pair to make sure they'll hold up. I don't normally use chamois creams, but will for anything over 200k.
The one fleche I did was a blast. It was very flat (I almost think it was downhill) so we were actually stalling for time. Killing time at the Waffle House.