In k3s, how can I promote a worker node to a master node?

I currently have a Raspberry Pi-based Kubernetes cluster based on k3s which, while originally a single-master cluster, has now grown to the point at which it is practical to have multiple (odd-number) master nodes.

My question is - with regard to k3s specifically - is if there is a simple path to promote an existing worker node to a master node, other than removing it completely and reinstalling it?

You cannot "promote" worker node to master node. There is not switch you could flip and from now on worker would become a master.

Unless you are already using external database, you cannot add more master nodes. If this is the case, you may need to reinstall the cluster.

Check external database options.

If you are already using external database, follow k3s documentation on how to setup HA cluster