Fortunately it hasn't happened very often, but we have had to make last minute changes due to breakdowns, weather, or health issues on occasion. Sometimes it meant canceling a reservation at whatever cost was involved, while other times it meant just shifting dates. For long trips to destination parks, we usually build in a couple of 2-3 day layovers along the way that can be used if necessary for schedule adjustments. Most private parks have been very accommodating when changes were needed. State and national parks have also worked with us when a delay prevented us from arriving on schedule. Yes, the "old days" when we could flip a coin to decide where to go tomorrow were great, but today is what it is, and we've adapted to it as best we can. We already have our reservations in place through next March. There likely will be some modifications before we get there, but we'll deal with them as needed.