Elasticsearch mapping issue for ES version 7.*

Hello,
We are facing some issue related to mapping when we ingest data.
Here is error we are getting:

"error"=>{"type"=>"illegal_argument_exception", "reason"=>"Mapper for [fn.keyword] conflicts with existing mapper:\n\tCannot update parameter [normalizer] from [custom_normalizer] to [default]"}}}

As the error suggests, you cannot change mapping for a field that already exists.

You need to look at the _mapping endpoint for the existing index to see what it's already set as. Then if you want to change it you will need to create a new index with the mapping you want and then reindex the older data into it.