I find the color pallette a little loud on the eyes, and the posts a little "Reader's Digest Large Print" for my tastes, but a forum is a forum. Functionally it's the same and the buttons are all either in the same place or in an easy-to-find new place, so it really doesn't matter to me. Frankly I found the old version a little clunky.
I also understand why upgrades are necessary: The bad guys never stop trying to hack their way in, and eventually older protocols prove to be irreparably insecure, so new protocols must be implemented. The old forum softwares don't support the new protocols, and patching them up to make them compatible would either be cost prohibitive or result in an unstable, unreliable mishmash that you'd hate even more than a new format. Sometimes it's just better to start with a clean slate, and while you're building a new product, why not incorporate a fresh new look like restyling a car, and address some user complaints about the previous interface?