Elasticache Global Datastore modify shards problem

I am trying to do is using global datastore for cross region replication. I have created Redis primary and secondary cluster in 2 regions with multi shards (elasticache_replication_group with cluster mode). They are control by global datastore. I need to scale up/down (resharding) when needed. Problem is shard is control by global datastore after created. I am not able to make the change on either primary or secondary. And for now aws_elasticache_global_replication_group is not providing any argument to do so.

Could anybody tell me is it anyway I can achieve that currently please? Or aws_elasticache_global_replication_group need to be enhanced to handle that? Appreciate it!