Upgrade data node to data + master node in production

I am running a production cluster with below configuration
No of nodes:6
memory size of each node=64GB
No of documents=1Billion
No of shards=5
No of replicas=1
All the 6 nodes are master + data nodes and there is no dedicated master only node.

However due to some reasons in the past only one of the node is configured as master and remaining nodes are configured as "node.master: false".
Now, I would like to make the cluster as highly available, so would like to convert 2 of existing nodes as master eligible to make 3 master node configuration.
So the question:
If we consider A,B,C,D,E,F as nodes, where A is master today, can we directly change config file of C and E and update "node.master:true" and restart the nodes C and E?

do the existing master node A should also be restarted post config changes done in C and E?

What version are you on?

Version is 7.10.1