Thanks for all your comforting answers. The reason I went directly to UA in this case was from good experience in dealing directly with TG on a similar question. In that case I called up TG reservations and asked them to book me on a later flight (no change in routing or dates) and they handled this very smoothly with my original booking updated as per checkmytrip.com.

Compared to the somewhat less responsive nature of my Bangkok travel agent (I've been using Traveller 2000 and it takes days to get an answer by email) working directly with the airlines just seems much faster. It also seems - though this is merely speculation - that availability is better when going through the airlines, ie. a CRWSTAR for instance is treated more like a normal D class ticket and is not subject to the same space regulations I have experienced with my travel agent. Is this something others have experienced as well or just a coincidence?