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

IMPORTANT: If you use {xpack} and are upgrading from a version prior to 6.3,
remove {xpack} before restarting: `bin/elasticsearch-plugin remove x-pack`. As
of 6.3, {xpack} is included in the default distribution. The node will fail to
start if the old {xpack} plugin is present.