We sailed on the 7-day in late September, 2000. It was only the sixth or seventh sailing on the 7-day, and the second sailing was diverted to Key West and Cozumel. The week that we were to leave, Hurricane Joyce formed in the Atlantic and was heading toward the Leeward Islands. I was pretty sure we'd be going to Key West and Cozumel at this point. Then, the day before we left, Hurricane Keith formed off the coast of Honduras and was headed straight for the Yucatan, so Cozumel was out.
When we boarded, we really didn't know where we would be going. We were told that we be in Key West on Sunday, and then after that, it all depended on the weather. By Saturday night, the captain announced at the welcome variety show that Joyce had been downgraded, and had turned toward the south. He said that while we would still stop at Key West, there was a good chance that we'd then be able to call at St. Thomas on Wednesday, as scheduled. We already had a tentative itinerary, with an extra day at
Castaway Cay, and a call at Nassau (we had already booked excursions on Nassau). On Sunday, when we left Key West, the Captain announced that we were indeed heading to St. Thomas. We called there and at Castaway Cay on Friday with no problems.
We never hit rough seas on this trip because the storms in question were never anywhere near the ship. The only rain we ran into was on Saturday night and early Sunday morning. By the time we got off the ship at Key West, it had stopped and the sun came out in the afternoon.
I'd say to go ahead and sail in the fall. You get the good discounts, and most of the time, the itinerary isn't altered. Just keep in mind that there is a chance of this happening, and go with the flow. Being on the ship is great fun in and of itself, and I'm sure you can find something fun to do no matter where the ship calls.