I'd call and check to be sure, but I'd think as long as you aren't changing the design of the deck (making it smaller or bigger) you wouldn't need one. But if you look like you are doing a lot of work and one of the inspectors drives by, he may think you're installing a new one and wonder why you don't have a permit. In my town most permits are $50, if I called and they told me I needed one, I'd go ahead and buy it so I wouldn't have to worry about it.