I have a small 3 node cluster running on NUC's. All 3 are masters with embedded etcd. I recently shut down one of them, as I had enough with two, and could spare on electricity. Running on two of three masters is in theory not ideal, but it works enough for this homelab. Things evolved and I might consider running all on 1, but would like to keep a second offline. Is it possible to convert a master node to a worker node, without having to fully reinstall? The node is running k3os, and I'd like to avoid having to reinstall it, for now.

As I understand it: if you had 3 server nodes in your High Availability cluster and you've turned off one you still have an HA cluster, because you have a quorum or working servers. If you turn off another one you don't have a quorum of working servers any more, so the cluster is now broken.

I know that. The idea is to convert to a single master cluster.

Okay. It's good you've said that, because I don't think it was clear in your initial question 🙂

You can use the backup and restore system to go from 3 nodes to 1 because you are using the embedded etcd. https://docs.k3s.io/backup-restore#backup-and-restore-with-embedded-etcd-datastore