My recent experience as an ROP Gold is excellent and never experienced an American carrier-style capacity control block in any class on TG metal, where there was an available seat. But I have also found ONLY the Bangkok main office can authorize the mileage upgrades, GUP's, Birthday Awards, etc.--the locals can't, to my knowledge. Usually, when I deal with the local (in my case, Beijing) TG office, they put the request in and it's confirmed within 24 hours. So, if you haven't already done so, phone the member desk directly in Bangkok, after first trying to upgrade online as suggested above.

There is a small possibility that for the date you want the upgrade, F is being held in unofficial reserve for travelling Thai VIPs or something.

I'd also like to know your outcome, as if they change procedures, this might cause me to reconsider TG ROP.