Frans,
I wanted to know this too and did a small test, a few days ago.
It seems that the installer is handling everything fine. It will install all wd12 dll's. It will even update HFCS to v12. (the structure of Hyperfiles itself hasn't been changed)
One thing is doesn't: removing all wd11 dll's. But that's not too bad. They are not in conflict with anything, but just consuming some disk space
Arie