If you read the letter very carefully, you can find what you should do: calling the National Customer Service Center to change the schedule. However, I can't garantee that you can change the schedule because the reason is not appropriate enough to change. Usually if an emergency has happened or someone died in your family, or someone in your family got hurt by accident, applicants are allowed to change their schedule. But your parents' case is different. So I don't know if you can change the shedule for your parents. But whether you can change the shedule for your parents or not, you need to call the USCIS National Customer Service Center, anyway. So just call them and ask what you can do for your parents. The phone # is 1-800-375-5283.