So, my sister and I were there last year when Irma came through. We were supposed to fly in on Saturday night, but realized that is when the hurricane would hit so our flights would likely be cancelled (and that was right, as the airport shut down around 5pm and didn't open until Tuesday afternoon). We ended up changing our flights so we arrived on the Thursday (and worked from the hotel room on Friday morning so as to not take extra vacation time).
For the parks themselves, they were EMPTY. Like seriously, no one there! It was actually pretty cool - if you wanted to go to Be Our Guest for dinner, you could have walked right up. And we did every ride in Magic Kingdom in 5 hrs on a Saturday afternoon. After the hurricane it continued to be extremely low crowds until the weekend, when people began arriving who hadn't needed to change their trips, plus other people who had just postponed by a week.
During the hurricane we stayed at Port Orleans French Quarter. Disney did a really good job overall. First, they stocked the rooms with flashlights, extra toilet paper, and extra trash bags, as they wouldn't be doing housekeeping for Sunday and Monday. They kept the food court open as long as possible, and while it was a limited menu (some pastas, pizza, burgers), it was very reasonably priced. They also had bagged meals you could buy to keep in the room. They also had characters out in the lobby to help keep the mood light and airy (Chip and Dale organized a dance party, and we also saw Pluto leading a Moana sing-along when it was on the TV, plus Donald doing a fashion show in the gift shop). I kept track via twitter of the goings-on at other resorts, and at Deluxe resorts with interior corridors, they kept the restaurants open (again a reduced menu though), plus did other activities in the lobbies. They also had a TV channel which had a constant loop of Disney movies. Many resort Cast Members stayed at the hotel for that weekend, so they were available to help out (and I think those people made $$ on overtime)
We were allowed to walk around the resort on Sunday morning and afternoon, but once the winds kicked in, they asked us to stick to our rooms. Overnight, it was definitely windy and rainy and we were kept awake a bit by the sounds outside, but we never lost power and never felt unsafe. The next morning we could see how crazy it was, especially at French Quarter, as the streets were just filled with tree debris and multiple trees had fallen down around the property. Thankfully there wasn't any major structural damage (there was a huge tree that if it had fallen in any other direction would have caused a lot of damage, but it happened to fall into a grassy lawn)! The cast members were out and working to clean everything up as quickly as possible, though they had asked people to remain indoors for the first half of the day just to keep safe. By evening the food courts were back open and we were able to walk around to Riverside and French Quarter. The next day the parks were back open and they added 1 hr EMH to Magic Kingdom, not that it was necessary due to low crowds!
Pre-hurricane, Disney also was very reasonable with cancellations. You could cancel the whole resort without any fee, and they weren't enforcing ADR no-show fees, as they knew people could be having IT issues getting in to cancel those. Airlines also allowed you to make changes without any fees (I don't remember if it also allowed cancellations with refunds, as that wasn't something we considered). If people had to extend their stays because their flights were cancelled, Disney offered significant discounts for the rooms (I think like 40%).
Overall, my advice would be: You can't really plan for a hurricane - you just don't know when or where it could hit. You can check the weather hurricane tracking in the weeks coming up to your trip and you may be able to see something developing (I followed Irma for weeks since it first started off the coast of Africa), but you won't know if it will actually affect you until much closer to the date. If you are very worried, you can get trip insurance. If I was going again in September, I would probably just plan as normal. Though maybe I'd consider staying at a resort with interior corridors so we wouldn't be confined to our rooms. And just be prepared to be flexible - know there are certain things out of your control and you just gotta roll with it!
