The alpha5 release note implies alpha5 indices are not compatible with GA:
IMPORTANT: This is an alpha release and is intended for testing purposes only. Indices created in this version will not be compatible with Elasticsearch 5.0.0 GA.
I tried upgrading form alpha5 to 5.1.1 and the old indices are gone. Is there officially a way to migrade alpha5 data to 5.1.1? Snapshot and restore seems promising but I'd like to ask for a quick answer before testing it out.