Hi Folks,
You are correct,
Upgrading to Junos OS with upgraded FreeBSD reformats the file system. Only specific files and directories are preserved unless precautions are taken. By default, the upgrade process only preserves the following directories:
/config
/etc/localtime
/var/db
/var/etc/master.passwd
/var/etc/inetd.conf
/var/etc/pam.conf
/var/etc/resolv.conf
/var/etc/syslog.conf
/var/etc/localtime
/var/etc/exports
/var/etc/extensions.allow
/var/preserve
/var/tmp/baseline-config.conf
/var/tmp/preinstall_boot_loader.conf
Moreover, we have to use no-validate to upgrade, request system software add package-name no-validate
Note: If you have important files in other directories, copy them from the router or switch to a secure location before upgrading the router or switch.
https://www.juniper.net/documentation/en_US/junos/topics/task/installation/junos-os-upgrading-kernel-freebsd10.html