Fall can be great! The only negative IMO is dealing with all the traffic at the height of Leaf Peeping season
Water levels can be lower, typically October is a drier month, but the latest climate predictions say that drought conditions should improve for East TN over the next few months. I hope they are correct. The heat has been tough and I am looking forward to cooler weather for work and my personal time as well.
As far as fishing techniques, I don't change up that much in the Smokies. Smaller/darker flies if the water is lower, nymphs (BHPT, Tellicos) are what I use most of the time. When in doubt, use a parachute adams (12-16), the female adams are a good choice as well.