You need to merge the etc configs do not ignore the etcupdate otherwise you will end up with non working configuration in /etc. After you generate the diff you can prepare the merge and install kernel then reboot and install the userland.
After that when you have working "freebsd-update" binary you can use it to install on top on that installation 13.1-RELEASE and continue to use binary updates instead of following the CURRENT version via src.
After that when you have working "freebsd-update" binary you can use it to install on top on that installation 13.1-RELEASE and continue to use binary updates instead of following the CURRENT version via src.