Auto expand replicas

I am trying to implement a scale up of elasticsearch cluster by increasing data nodes.

Data nodes: 10 nodes
1 node: 2 shards (index1 and index2)
Index1: 10 Primary and 0 replica
Index2: 1 Primary and 9 Replica

I want to make it 20 nodes and I am applying auto_expand_replicas in index2. Sometimes it cause the cluster in yellow state. Will it affect the operations. How to avoid such events ?

I thought of first adding the nodes and then add replicas but I want the above process as it is triggered automatically according to the CPU usage. (I am trying to add autoscaling)