Can I relabel /, /home, and /boot/efi partitions without problems?
Solution 1:
Yes you can label or relabel your partitions as long as you do not use labels in /etc/fstab. You will have to use a live USB. I just labeled my root partition.
It is a good way to know what is what when working with a partition editor.
P.S. I tried again without a live USB: success using gnome-disks.