Scaling with regard to Nested vs Parent/Child updates

I'm running a proof of concept for us to run nested queries on more "normalised" data in ES.

e.g. with nested

Customer ->
- name
- email
- events ->
- created
- type

Now I have a situation where a list of events for a given customer can be moved to another customer. e.g. Customer A has 50 events
Customer B has 5000 events

I now want to move all events from customer A into Customer B

At scale with millions of customers and queries are run on this for graphs in a UI is Parent/Child more suitable or should nested be able to handle it?

What are the pros and cons in my situation?