Just to give an update regarding this problem.

I first called the local customer service here in Romania and the lady that answered saw there was an issue which didn't allow my ticket to be upgraded, but confirmed that this wasn't because of an availability problem.

Then I called DE customer service (during DE office hours) and the first guy that answered definitely wasn't from Germany (judging by the way he talked, I speak fluent German) and kept on saying that there is simply no availability. At one point he wouldn't believe me that I am the passenger and not a travel agent when I kept telling him that availability is not an issue, yuck...

Second call to the DE customer service got me to a representative that was from Germany at least and even though at first the answer was no availability, after some more digging, they came back with prices for upgrades on all segments. Still, it wouldn't be possible for me to upgrade through the website/app, which is what I wanted (so that I can track the prices and hopefully get a lower price if a lower class opens up).

Oh well, I give up, I'll just bid and hope for the best. Thanks for your suggestions.