That's not true. I'm not exactly sure how it works, but if your start on the A320 you can transition to the A340 or the B767 as a FO, then go back to the A320 as a Captain, and then transition again to the A340/B767 as Captain. A friend of mine started as a A320 FO in LAN Express, then he went to the B767, and now he is a year or so away from becoming a Captain in the A320.
Maybe it has something to do with which company hires you (my guess would be if you ared hired by LAN Airlines you transition to the A340, if hired by LAN Express you transition the B767 and so on).
Regards.