get the whole thing out. The 5.25's that are in there have an oversized frame, so (assuming your mounting points are similar), the 6.5's will line right up. But, the hole for the magnet and casting won't be big enough. try to push it in and see where it hits. Dremel that area out. Keep doing that until the 6.5 fits. It's a crude method, but I think that's what everyone's done. Pretty easy, just takes a little time.
