Renaming Values

[Tomo_M (Tomohiro Mitani)]

Though I'm not sure these are only ways, you may use runtime fields to manipulate the values.

Or of cource you may do it outside elasticsearch while indexing.

Ingest pipeline : Getting lookup values from another index

No this is not possible and should be done outside elasticsearch index process. Logstash for example. I wrote an example of this here: http://david.pilato.fr/blog/2019/01/10/from-a-startup-to-a-listed-company-6-years-of-fun/