With partner awards, it is best to think of it as United buying a revenue ticket from the partner for you with their own money, and then keeping your miles for themselves. Treat it like you would treat it a ticket purchased from a travel agent, only your travel agent is United. The agents at Thai might not even be able to modify the ticket, since United issued it. And even if they can, Thai has no obligation to rebook you if you don't show up to the first flight.

You're best off calling United right now and changing the award ticket to something that you know will work for you.