3544quebec -

The "D UP" you saw on your boarding pass does not mean "D Upgrade". It means "DUPE" -which is the airline abbreviation for "DUPLICATE BOARDING PASS".

From knowledge - THAI will only upgrade when the section you are seated in is oversold by the time you had checked in.

If you had connected from a connecting flight and have already been issued a boarding pass, you will not get upgraded because you already hold a seat. Passengers who check in late (that's a clue!) and are GOLD star alliance members will be upgraded as required.

Besides, THAI's business class is nothing to rave about.