Solutions
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Soulflight's post in FSX Reinstall impossible "You cannot upgrade between..." was marked as the answerHello again,
I got back to my home now and looked the Product Code of the german pro version up: {F535B2CF-C9BB-4162-B03A-02D6971F32CC}. Done a Registry search, found some entries and deleted them. Someone should publish such Product ID's just in case people run into the same issue. Installer runs normally now.
Make sure you still have access to the Setup.ini of any FSX CDs you use for installation. Or save a copy.
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Soulflight's post in All "Center" ATC stations gone was marked as the answerNevermind, there were some bgl-files missing, I got replacement from my disk.