gpart has functionality to change the label of a (GPT) partition. This functionality works like it should, however, after a label change the /dev/gpt/ entries remain unchanged. glabel status output remains unchanged. The change only takes effect after a reboot.
The device was not in use or otherwise 'locked', gpart list shows the new label, but otherwise it appears that the gpart modify -l command does not actually change the device entries, as is expected and desired.