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
{
            "index_patterns": ["k8s*"],
            "template": {
                "settings":{
                    "index": {
                        "number_of_replicas": 0,
                        "number_of_shards": 2
                    }
                },
                "mappings":{
                    "properties": {
                        "@timestamp" : {
                            "type": "date"
                        },
                        "metricset": {
                            "type": "keyword"
                        },
                        "k8s": {
                            "properties": {
                                "pod": {
                                    "properties": {
                                        "uid": {
                                            "type": "keyword"
                                        },
                                        "name": {
                                            "type": "keyword"
                                        },
                                        "ip": {
                                            "type": "ip"
                                        },