Best way to re-index documents

Hi, i have a huge database Elastic search and my goal is to optimize and change the mapping of documents inside.

What's the best way to do the re-indexation ? and how i am sure that all documents are well index ?

I used logstash but it's not log the error when document not inserted

Best regards,

If you have elasticsearch 2.3 at least you can use the reindex API.