October is still low season so you will probably get an upgrade. (I usually do.) Also it doesn't hurt to ask for extras (spa treatments, breakfasts, etc.) before you confirm your reservation. They're usually very helpful when they are not fully booked.

The only thing I would point out is that the beach at Trisara will not be swimmable in October. So if you want to swim in the ocean, you'll need to go by car. There are a couple of decent (but not very private) bits of beach that aren't far away.