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.
"Binary upgrades between RELEASE versions (and snapshots of the various security branches) are supported using the freebsd-update(8) utility. The binary upgrade procedure will update unmodified userland utilities, as well as unmodified GENERIC kernels distributed as a part of an official FreeBSD release. The freebsd-update(8) utility requires that the host being upgraded have Internet connectivity."
So you have to read the manpage of freebsd-update, understand it, and then find the version name in another document... After a while I think that this is it :