I was actually surprised that it could not be any easier to use Qcredits to upgrade at the airport in BKK. They knew immediately what I was talking about and it was done in about one Minute (some typing, a couple if questions if I really really really sure I do want to use my QC).
Why should it be any more difficult to upgrade the second sector in BRU? They can see and access the whole booking on their computer and should know what Qcredits are.
(all the time I was checking before there were no upgrades available online/ but free 1st class seats to book with cash)