During a rolling upgrade, primary shards assigned to a node with the higher version will never have their replicas assigned to a node with the lower version, because the newer version may have a different data format which is not understood by the older version.

If it is not possible to assign the replica shards to another node with the higher version — e.g. if there is only one node with the higher version in the cluster — then the replica shards will remain unassigned and the cluster health will remain status yellow

In this case, check that there are no initializing or relocating shards (the init and relo columns) before proceding.

As soon as another node is upgraded, the replicas should be assigned and the cluster health will reach status green.