you need the configuration from your mongoDB to be able to read the data in elasticsearch.
The easiest option would be a copy of your mongoDB database, use that as starter for your new cluster and connect both elasticsearch nodes to each other that you can copy the data from the old to the new server.
This is not an easy task - as in elasticsearch the information who has access to what is stored together with your data. If the meta information change (cluster information, userid, streamid and more) you are not able to access the data again from within Graylog.