How to roll back from ES 6.4.3 to 5.6.12 - using snapshot & restore

[juiip]

I took a snapshot of my cluster in version 5.6.12, upgraded to 6.4.3 but now want to move back to 5.6.12...
Is it possible to roll back by restoring this snapshot?

I tried running the following command in Kibana:

POST /_snapshot/my_backup/snapshot_07-11-2018/_restore
{
  "include_global_state": true
}

However I get the following error and don't know what it means?

{
  "error": {
    "root_cause": [
      {
        "type": "invalid_index_name_exception",
        "reason": "Invalid index name [.triggered_watches], already exists as alias",
        "index_uuid": "_na_",
        "index": ".triggered_watches"
      }
    ],
    "type": "invalid_index_name_exception",
    "reason": "Invalid index name [.triggered_watches], already exists as alias",
    "index_uuid": "_na_",
    "index": ".triggered_watches"
  },
  "status": 400
}