If you are 99 days or more away from your departure date, you can change to any other tour and date within the same travel season. If you have purchased travel protection you can change three times for no penalty. If you have not purchased travel protection (do it!) there will be a $50 fee with each change. You will be subject to pay the difference in price between tours, if any.
After the 99 day mark, additional fees can incur. Read more about changing your trip in the Booking Conditions.