A DB counter agent once told me that it is possible to upgrade a saver ticket to the normal price by paying the fare difference, so the ticket is not totally lost. But you have to find an agent who is willing and competent enough to it. That you will certainly NOT find at Düsseldorf airport, that counter is dreadful and really sucks,,a disgrace to DB. But Duisburg or Düsseldorf main station would do it, I reckon.