It won't cost more points, but there may not be availability for the Boardwalk view at this time. THose do tend to book rather quickly.
If it were me, I'd call the owner and ask him/her to check and change if possible. You could also request that the owner arrange to waitlist for the Boardwalk view. I'd suggest you waitlist for the entire reservation so that you wouldn't run the risk of having to move between the Pool/Garden view room and the Boardwalk view room. If you list for the entire reservation to come through at once, you either get the BW view for your entire stay or you don't. THere is less chance of getting it this way, but at least you won't have to move back and forth.
FWIW, the BW views are nice, but there are lots of very nice Pool/garden views, too.
Best wishes -