Hi Maggie, the aufs to overlay migration takes place with balenaOS 2.84.0, so it will happen as device types get updated. The engine will then migrate applications from aufs to overlay avoiding the need to re-download containers.
The affected device types are:
beaglebone-black
intel-edison
intel-nuc
odroid-c1
raspberry-pi
raspberry-pi2
raspberrypi3
raspberrypi3-64
fincm3
revpi-core-3
npe-x500-m3
up-board
Hope that helps.