When we went in early November 2002 the resort decoration were going up. I have heard of the overnight transformation following Thanksgiving before, but I don't think it is a complete transformation. I've also heard that the transformation from the day before to the first day of the flower and garden festival is amazing, but we were in Epcot the day before and the day of and it wasn't that different. A bit more color, but the big things had been done the week before.
PS - Thanks for the update on the Christmas decorations to the OP. I'm so excited. We've never been at full blown Christmas before and we leave in 25 days