One item of note, your ticket must be booked through Thai. I'm flying next week on an F award booked through United NRT-BKK-DPS, the BKK-DPS is on a 744 but sold as J. I reached out to reservations about purchasing an "upgrade" to the wide-open F cabin, here was there response:
Kindly be informed that preferred seat purchase can be proceeded on ticket issued by THAI Airways
(Document 217-xxx xxx xxxx)only.
This service cannot be offered since your ticket was issued by United Airlines (Document 016-xxx xxx xxxx).
Further information or assistance needed, please do not hesitate to let us know.
I'll try when I get to the airport and see if they will move us up for free or if they charge, assuming there is availability still.