Even 4 months out, I've been able to change my January's reservations to other resorts and room types. This isn't for Marathon weekend, but I do stay in WDW for just about all of January.
I feel very lucky that in August, I was able to change a December reservation from VWL to OKW because my schedule changed drastically. I could not have gone into VWL from OKW at that time, but what I got is fine with me. I went from the early part of December into the slightly higher points of the second part of December but before the holiday high point time.
For F&W, it's not only the time, but the resorts you want to get into. People here are so happy that they got BWV or BCV at 7 months, because it does happen and those resorts are so convenient for F&W. I think people also have the lucky feeling that could get into BLT during that time because of the Halloween parties, and later the Christmas parties.
There is some movement, and if you go for a 1 bedroom, it seems you are a bit more likely to get what you want at 7 months even for the most poplular DVC times.
Still, I would buy where I wanted to stay and use the advantage whenever I can. I own at 7 resorts because I know what I like when I use my DVC membership. This December is the first time I won't be at my first choice, but still, it was August, and I was able to get a 2 bedroom at OKW, I am thrilled. I book NYE time at 11 months exactly, and I'm at BWV then!