As far as I know, the rulebook lists are still legal if both players are using them. The army books contain signifigant changes i.e. M8/M9 are counted against a squad's special weapon options and the Tanker's spit is now 12" instaed of 18".
Ya, it'd basically be like using outdated codexes in 40k. You can still use them as long as your opponent agrees. This isn't a game where anyone's going to say, "No, those rules are old! You can't play with them ANYWHERE!"