In Warhammer fantasy, is it permissible to make an army using an old version of a race's codec?
Every tournament has its own specific rules, but I've never seen one that hasn't required the use of the latest/greatest rules sets available, both for the game itself and the army. There are often additional restrictions, e.g. banned items or characters, troop limits etc, as well as model requirements (minimal painting requirements, no use of non-GW models, etc).

There are several good reasons to require the latest rules for a tournament:


Everyone is on a level playing field (tournament organiser understands the environment)
Everyone knows exactly what to expect
Army list checking and meta game preparation is much simpler
Strange, unbalanced or just broken loopholes from previous editions are obsolete
Games Workshop gets you to spend more money