They can be sticklers for rules. However, sometimes you get a really good rep who will help. I had a ticket to a popular show that I found out I couldn't attend because I was having surgery. I only gave them like two days notice, but they managed to cancel that ticket and get me a ticket to the same show a couple days after I was out.
I would try explaining that it is for a medical condition. As the previous poster said at least they might be able to past date the ticket. Also, check to see what "types" of "insurances" your credit or debit card might have that you purchased the ticket on. (You would be surprised what some cards cover.)