I tried this, but it results in an infinite loop of "The following file will be removed, as it no longer exists in FreeBSD" that asks to remove apparently all files from /etc, one by one.
Is this system in a jail? Have you already attempted an upgrade? If so you could rollback and start from scratch via # freebsd-update rollback; a reboot is needed afterwards.
# freebsd-update install
Which may ask to manually merge some configuration files or accept the automatic merge.
Reboot after new kernel has been applied
# shutdown -r now
Run freebsd-update to update userland components
# freebsd-update install
Rebuild and reinstall applications and reboot into the new userland
# shutdown -r now
Remove stale files
# freebsd-update install