elasticsearch/docs/reference/upgrade/remove-xpack.asciidoc

IMPORTANT: If you are upgrading from a version prior to 6.3 and use {xpack}
then you must remove the {xpack} plugin before upgrading with
`bin/elasticsearch-plugin remove x-pack`. As of 6.3, {xpack} is included in
the default distribution so make sure to upgrade to that one. If you upgrade
without removing the {xpack} plugin first the node will fail to start. If you
did not remove the {xpack} plugin and the node fails to start then you must
downgrade to your previous version, remove {xpack}, and then upgrade again.
In general downgrading is not supported but in this particular case it is.