Currently it is possible to migrate an existing data stream to a time series data stream.
This is done by updating the index template that created the data stream. 
Then after a rollover the data stream is a time series data stream. 

Via a similar way it should be possible to change time series data stream back 
to a regular data stream. Currently when changing the `index.mode` setting
on an index template from `time_series` to `standard` (or removing the setting)
results in a validation error.
modules/data-streams/src/javaRestTest/java/org/elasticsearch/datastreams/TsdbDataStreamRestIT.java
"number_of_shards": 2,
                        "mode": "time_series"
                    }
                },
                "mappings":{
                    "dynamic_templates": [
                        {
                            "labels": {
                                "path_match": "pod.labels.*",
                                "mapping": {
                                    "type": "keyword",
                                    "time_series_dimension": true
                                }
                            }
                        }
                    ],
                    "properties": {
                        "@timestamp" : {
                            "type": "date"
                        },
                        "metricset": {
                            "type": "keyword",
                            "time_series_dimension": true
                        },
                        "k8s": {
                            "properties": {
                                "pod": {
                                    "properties": {
                                        "uid": {
                                            "type": "keyword",