Based on my experience with Irma, they won't switch you for another date, but there will be a lot of people cancelling their trips which will open up a ton of dining reservations and fastpasses in the couple days following the storm. It's no guarantee you will be able to get another reservation for Oga's on a day the parks are open, but your odds are better than if you needed to reschedule for another reason.
When we were there during Irma, we had to switch almost all of our park days around due to the parks being closed for 2 days. I was able to get every dining reservation I wanted in the 5 days following the storm. I was able to reschedule our fastpasses and got everything I wanted. You just have to hope that people cancelling their trips cancel their fastpasses in addition to their dining reservations.