Hey Guys,
I have written up my experiences with upgrading a few elasticsearch clusters from V1.X to V6.X over at github.
I've included a docker-compose with elastic 1.7.5 (with kibana + sense) & 6.2.4 (with kibana and dev_tools) so you can run some re-indexing tests quickly and easily.
I have also included a docker-compose for a logstash container which can be used to apply templates to the new cluster and move the data in a simple docker-compose up
command.