In recent testing I discovered that it is impossible to downgrade/rollback to a previous version of LXD if the upgrade is unsuccessful. This is because the LXD database is upgraded, and no facility is provided by LXD to move it back to an earlier version. The LXD developers have clearly said that this downgrade functionality is UNSUPPORTED.

This is a horrible quality of a containerization system that is supposed to be used in production.

This is what happens when a downgrade is attempted:

Thanks, LXD.

The funtoo ebuild can (and should) "work around" this by backing up the LXD database prior to any upgrade automatically, and providing pkg_postinst() instructions on performing a rollback in the case that for some reason the new version of LXD is determined to not function properly.

References:

https://discuss.linuxcontainers.org/t/downgrade-lxd-from-snap-from-edge-to-stable/5217

"LXD downgrades are not supported." ^^