I understand they can keep moving around until the 34-35 week mark, especially if you have a bit more room (due to longer torso, smaller bub, more amni fluid). Mine was sitting breech at 31 weeks but by 33 weeks had tipped into heads down. Even if they are lying breech after that a good percentage of bubs manage to get themselves head down anyway.

Some of the other ladies who post will have some tips on best posture to encourage good positioning.