Perhaps I am missing something.
You can dynmically change the number of replicas see here
Dynamic index settings
Below is a list of all dynamic index settings that are not associated with any specific index module:
The number of replicas each primary shard has. Defaults to 1.
Example : Replicas are dynamic, meaning it can be set after the index is created, number of shards is static and can only be set at creation.
If you have 5 shards and 1 replica you will have 5 primaries and 5 Replica Shards (1 replica for each Primary)
If you have 5 shards and 2 replicaa you will have 5 primaries and 10 Replica Shards (2 replicas for each Primary)
You can change replicas with this