Babies can flip all the time -- even in labor!! While it gives us mamas peace of mind knowing our babies are more optimally positioned, 31 weeks leaves PLENTY of time to turn still. The Webster technique by a chiro can be helpful as it loosens up the RL joints to encourage more optimal positioning, but doesn't guarantee (I know many will go in for a Webster adjustment before an external version if labor is really RIGHT around the corner... makes version more successful as far as I read). Spinningbabies.com has lots of ways to help encourage optimal positioning as well.
I sometimes have peeked at the June DDC (because an in-town friend of mine is on there... just had her baby last week!) and there are posts of babies flipping at 39+ weeks. Not sure that's really all that reassuring, but just saying that it happens. Our babies like to be on the move :)