About 2 weeks ago, Thai Airways changed their reservations system to Amadeus. The first hint was that the original 6-digit Thai Airways PNR for a future booking of mine using United miles didn’t work on the Thai Airways website. Some on FlyerTalk have reported that old Thai reservation codes still work, while others have reported that they needed different reservation codes than the ones they originally had in order to check their bookings.
Whatever the case, I recommend double checking the Thai PNR for any reservations you have with Thai Airways, especially if they were booked with United or US Airways miles. You can call United at 1-800-UNITED-1 or US Airways at 1-800-428-4322 to confirm the PNR for your Thai Airways flights. You can also call Thai Airways directly at 1-800-426-5204.
One interesting tidbit is that the itinerary I have also has segments on Lufthansa, which also uses the Amadeus reservation system. I was able to use my 6-digit Thai PNR on the Lufthansa website and select seats on my Thai Airways flight.