Scotch,

Suggest you ask the Crew when you board what they suggest. You might be pleasantly surprised and the lav(s) may have a changing station. Ask the Crew what they suggest if the need arises, when you board. We've had good luck on some flights and not had to change our son until we landed. A wet diaper doesn't always require immediate changing. Heck, most babies sleep through the night with the same diaper on. Personally, I would avoid changing poopy diapers in a row of seats. It is awkward for you and fellow passengers. Make sure to do a last minute change prior to boarding the aircraft.