Both my last two (my vba2c's) were bum down until about 38 weeks, they both flipped after my 38 week appointment with my midwife.

I did half handstands in my shower and some squats afterwards and it worked both times. My theory was the half handstand (just bending over and putting your hands on the floor and up on your tippy toes) was that the baby would float up out of my pelvis and then flip over the right way, and the squats opened my pelvis up so bubs would stay put. I also went on all fours and rocked back and forth and then did more squats.

Dunno if my theory is correct for everyone else but it worked both times for me!