No personal experience, but here is the full version from DC:
"You can easily shift reservations you’ve made, up to 48 hours prior to your reservation date, to any future date through 2022. To accommodate this, we will allow one complimentary date change prior to your visit, pending availability. Further changes to your itinerary will be provided for a nominal charge of $50 per change. Bookings made within 14 days of original reservation date will be booked at the original rate. Bookings made outside of that time will be subject to change based on seasonal rates for the new date chosen. If you decide to cancel more than 30 days prior to your reservation date, you will receive a 100% refund."
Which is only fair. Otherwise people would book the cheapest Jan date and change it to July for free....