For BKK-ICN, have a look at this:
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Looks like you're able to upgrade to an F seat, but with C service. For the opposite direction, ICN-BKK, I don't see it being offered, but they do offer something called Royal Business Seat on the Asian routes using a B747, which is basically the same thing. So perhaps they will offer it in the ICN-BKK direction also, but haven't publicized it yet. Or perhaps they'll only use it if C is overbooked.