Hey all
I just had a call with TK to use my upgrade vouchers for a flight from BRU to ICN and it turned out to be a rather smooth experience. I had first booked the ticket with a free "hold the price" option, but the agent could not use my vouchers to upgrade. He told me to make sure to check if there was availability in J before paying. I had already checked and there was, so he adviced me to pay the ticket and call back, telling me he couldn't guarantee it but if there was availability in J there was a very high change of having a succesfull upgrade. He also said that if I didn't get the upgrade now, I should call back from time to time, to check if places had become available. So I paid the ticket, called back, and was lucky enough to get the same agent. And a few minutes later the whole route was upgraded. Great success!

Shoutout to the agent btw, perfect English and he knew all about the vouchers and how to use them. He also gave me the rather welcome message that I had four vouchers, so I now still have 2 left.