I don't know if you can go from 5 to 7, however if you get the dazbook it contains a free copy of Carrara 6 Pro, that can be upgraded to version 7 from what I understand.  There are also a couple of magazines that had it but I don't know if you could find those.  Worse case you could call daz ask them about the 5 to 7 upgrade if not just order the book and tell them you want to upgrade that version 6 to 7 it will cost you 30 or so more but would get you to a full version 7.  It appears to be on sale, don't forget if your  platinum member  you will get an additional discount.