Your approach should be fine. If you don’t want any downtime during the process, your may want to make sure each index has a replica, patch index nodes one at a time and wait for its indexes to be online before taking down the next one.
Also note that, in versions 5.0 or higher, index service supports swap rebalance. So if you were to swap in a patched node and swap out an old node, the indexes would get automatically transferred onto the incoming node. However, it is always faster to stop the index service(or failover the node), patch it and restart the service(or recover the node).