Increase number of shards for an existing data stream

Hi,

I have been using the _split API to change the number of shards on an existing index. This has been a simple task and has worked as expected.

I have not had much success doing the same for a data stream which may also contain a number of rolled-over indices (managed by ILM).

Are there any pointers for how to achieve this? I don't want to delete the existing data which would be an easy solution.

Thanks.