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/test/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProviderTests.java
now,
                settings,
                null
            )
        );
        assertThat(
            e.getMessage(),
            equalTo(
                Strings.format(
                    "backing index [%s] in tsdb mode doesn't have the [index.time_series.end_time] index setting",
                    DataStream.getDefaultBackingIndexName(dataStreamName, 1, twoHoursAgo.toEpochMilli())
                )
            )
        );
    }
    public void testGetAdditionalIndexSettingsNonTsdbTemplate() {
        Metadata metadata = Metadata.EMPTY_METADATA;
        String dataStreamName = "logs-app1";
        Settings settings = Settings.EMPTY;
        Settings result = provider.getAdditionalIndexSettings(
            DataStream.getDefaultBackingIndexName(dataStreamName, 1),
            dataStreamName,
            false,
            metadata,
            Instant.ofEpochMilli(1L),
            settings,
            null
        );
        assertThat(result.size(), equalTo(0));
    }