I never had a deal with scripting and API.
I have one ELK on-demand instance and for example there are 10 clients and all indices have format aka "logstash-2019.01.01-000001".
How to build script that will reindex data and I can choose what client data should be reindexed?
I am not sure I fully got the question, so feel free to expand (for example with a sample dataset), but you could use the reindex API in combination with a query that filters for your clients data and then specify a different index for each of your clients.
Hope this helps!