Comment 31 for bug 1629540

A fresh install shouldn't trigger the migration script at all, so you should be fine.

It'd however be interesting to know if the following works for you then:
 - lxc network create lxdbr0
 - lxc network attach-profile lxdbr0 default eth0

If that doesn't blow up, then it'd match what Daniel reported, where apparently creating a new bridge with IPv6 enabled (as is our default) does work...