I refuse to believe that GW will ever outright invalidate army/codex books with an edition change again. It simply isn't good business. It would also mean that the rules would change so massively that old rules don't function any more, and I don't think that they would perform a wholesale reset of the game again. I think there are elements of the previous editions that need to come back to create the right mix, but they are doing that slowly with 40K now without resetting the game. The best solution is to get as close as possible, make one or two armies with old books crap in the game, and if necessary, release WD lists to kill off a book that just doesn't work(like with Codex:Witchhunters).