I have upgraded to Ubuntu server 20.04 Focal from 16.04 Xenial and removed the LXD and LXC packages already before.
Now with the LXD snap installed lxd.migrate gives me:
=> Connecting to source server
error: Unable to connect to the source LXD: Get "http://unix.socket/1.0": dial unix /var/lib/lxd/unix.socket: connect: connection refused
Trying to install the apt version of lxd leads to the same result.
Excellent, that’s definitely a better approach than doing it fully by hand as it’s easy to overlook something lxd.migrate does (like mangling zfs mounts, …) and end up in a bit of a confused state.