Unfortunately you are not covered by EC261/2004. The regulation only covers flights departing from EU or on any flight operated by an EU airline. There is no point trying to use EC261/2004 because it is irrelevant in your case.

IMO this is purely the fault of the ticketing airline and has nothing to do with TG. TG was paid by the ticketing airline for flying you in business class. You're ticketing airline should have notified you about the downgrade in advance and should have offered alternatives. You should contact the ticketing airline and ask about compensation (your chance of getting anything more than the points difference for that leg is pretty low IMO). Thai did nothing wrong and only received money for business so I wouldn't waste my time trying to get anything from them.