The weather in October can vary wildly - in Oct. 2009, I was there for 11 days. We had both record-high temps (low-to-mid 90s with high humidity the beginning of the week) and record low temps, all in the same trip!
I didn't bring nearly enough clothes for the cooler weather - I ended up frantically searching for warm socks at Epcot one day, when there was a really cold breeze coming off the lake. I was wearing sweapants, but the cool breeze went right up my calves and I was cold! Finally found a pair of pseudo-knee-socks at the Japan pavilion, which went part way up my calves. And I only had one pair of long pants and one sweatshirt. Brrr.
So my advice is to change the long-range forecast before you leave. (My problem was, the forecast I saw didn't go beyond the first week, which was unusually hot, so I brought only short sleeve tops and shorts. When the weather dropped 30 degrees from one day to the next, I didn't have the right stuff!)
To be safe, bring a little bit of everything (long pants, shorts, sweatshirt, jacket, sleeveless tops). You can always layer!