We are also going to visit with old friends in Cody so this is only a change in WHEN we go to Cody. We have a full month planned in Yellowstone/Grand Teton and Glacier so I'm not worried about letting them pick the activities a few days. I'm a organismal biologist (with an undergrad background in geology) so they have had no choice but to experience the wonders of nature 365 days a year for the past 11 years ;) - sometimes they just want to do "normal" things. Believe me they will be tired of having to listen to Mom's prattling on about trophic cascades and keystone predators or the Yellowstone caldera by that point in the trip and need a break.
That said, would it be any advantage to deal with Fishing Bridge for a single night on Sunday and campground hunt Monday morning or should I just go looking for space in a unreservable campground Sunday afternoon? I have no idea how a holiday weekend will change the campground migration patterns.