Layer your clothes. That way, as it warms up, you will be comfortable. When we go next week, I will probably wear capris, shorts or pants with a t-shirt and a sweatshirt or sweater.
I live about 45 minutes from WDW, and there are many days in the winter months where it will be in the 40-50's in the morning and warm up to the 70s-80s.
Hope that helps!