Hi Analog-X64. I got my copy of C6Pro clear back with the first issue of 3DArtist (14 issues ago) and I also had a copy of it from 3DWorld, which I gave to a friend here. The serial number on both discs was the same! I described how I was able to do the upgrade here in this thread.
http://www.renderosity.com/mod/forumpro/showthread.php?thread_id=2794641&page=1
I don't know if DAZ is offering the same upgrade deal at this time.