You can always use old models. I routinely play against an army comprised of old-pattern Rhinos and RTB01 beaky Space Marines. An ork is an ork, even if he's a cookie cutter plastic or the newest release. Even Games Workshop won't tell you you have to have the newest models in a tournament setting-- it's unusual to see since a lot of people build armies for specific high-profile things, but for Rogue Trader events, it's not uncommon to see old stuff.