I would like to change the NID on a 2.1.6 combined
MGS/MDS but the 2.x isn't clear how to do this.

The documentation describes what to do on a MDS.

Is this necessary  on a combined MGS/MDS?
When the device is mounted, will it automatically acquire the new NID, since it is an MGS as well?

mds-node$ tunefs.lustre --writeconf <device>
mds-node$ tunefs.lustre --erase-param --mgsnode=<new_nid> --writeconf <device>