I didn't find the api that returned the field using the alias。I think es should support returning fields using aliases。
The thing is that elasticsearch always returns the _source document as it was sent initially. If you sent it using an old field name, it will be returned as is.
We never transform it.
But if you use https://www.elastic.co/guide/en/elasticsearch/reference/current/search-request-stored-fields.html you can probably ask elasticsearch to return the exact fields you want (the new ones). Only if you have stored them though.