Just to finalize my experience:
After 63 downloads and installation of Win8 updates the store finally showed the 8.1 upgrade. The whole process of downloading and installing to 8.1 took about 3 hours but now I am running 8.1
For me setting the win8 partition to Active did the trick but I had to start from scratch.
Now the long road of re-installing all my software again begins. At least I get rid of some accumulated clutter :-)