TG changed its LAX-BKK flight to a 340-500 a/c, which on TG offers only Business, Premium Economy, and Economy.

If you have a full-fare ticket (booking class F), you have a full range of options: you can cancel the ticket for a refund, you can rebook the LAX-BKK portion via any airline that offers 1st class (though this will require a connection), or you can take the TG non-stop in Business class. If you take the TG in J, it's possible that your ticket price will be reduced some -- you'd have to contact Travelocity.