As far as I know, the revised Kramnik series contains not only updated theory but major repertoire changes, in especially against QGD a change from classical Bg5 lines to Catalan. Of course this also affects early move orders against Symmetrical English, Semi-Tarrasch etc. [I can´t find my source but acc. to my memory the main point was 1.Nf3 Nf6 2.c4 e6 and now 3.g3 (new) instead of 3.Nc3 (old) ?!]
The differences between old vol.1 and new vol.1a+b are not very big (surely a good update!), but from volume 2 on the old and new versions are incompatible to a certain/large extent. If you not only pick some interesting parts out of the books but want to follow the proposed rep very closely you´ll surely get holes if you mix old and new volumes!. I read/heard somewhere that the new series is planned to have 7 volumes (??), but wouldn´t bet on any dates for the missing volumes. Maybe the chess book market just doesn´t need another high-quality Catalan book ...

tracke 

Here's a pertinent link:

http://www.chesspub.com/cgi-bin/yabb2/YaBB.pl?num=1285257416/1#1

Also, if you search for "OFWAK", you'll find quite a bit about these books here.