In my Sundance, I upgraded from the original 32" TV to a 39". I faced the same problem you are having, in that the TV opening was too small. My solution may or may not work for you based on the location of your TV cabinet. (Mine is totally out of the way.)
I first located all the backing on my cabinet to ensure I had enough support. I then cut a piece of 3/4 finish plywood and made wood brackets that were positioned on the cabinet backing to stand off the new plywood from the back of the existing cabinet. I then attached the plywood to these (all stained and finished to match). On the new plywood backing I mounted the existing swivel (locking) bracket. In the end, the new TV stood out proud of the existing opening by about 2.5 inches. Once again, since my cabinet is totally out of the way, this was not a problem.
To quote a well used cliché, you have to think outside the box.