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/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
} else if (dataStream != null) {
                indexMode = dataStream.getIndexMode();
            } else if (timeSeries) {
                indexMode = IndexMode.TIME_SERIES;
            } else {
                indexMode = null;
            }
            if (indexMode != null) {
                if (indexMode == IndexMode.TIME_SERIES) {
                    Settings.Builder builder = Settings.builder();
                    TimeValue lookAheadTime = DataStreamsPlugin.LOOK_AHEAD_TIME.get(allSettings);
                    final Instant start;
                    final Instant end;
                    if (dataStream == null || migrating) {
                        start = resolvedAt.minusMillis(lookAheadTime.getMillis()).truncatedTo(ChronoUnit.SECONDS);
                        end = resolvedAt.plusMillis(lookAheadTime.getMillis()).truncatedTo(ChronoUnit.SECONDS);
                    } else {
                        IndexMetadata currentLatestBackingIndex = metadata.index(dataStream.getWriteIndex());
                        if (currentLatestBackingIndex.getSettings().hasValue(IndexSettings.TIME_SERIES_END_TIME.getKey()) == false) {
                            throw new IllegalStateException(
                                String.format(
                                    Locale.ROOT,
                                    "backing index [%s] in tsdb mode doesn't have the [%s] index setting",
                                    currentLatestBackingIndex.getIndex().getName(),
                                    IndexSettings.TIME_SERIES_END_TIME.getKey()
                                )
                            );
                        }
                        start = IndexSettings.TIME_SERIES_END_TIME.get(currentLatestBackingIndex.getSettings());
                        if (start.isAfter(resolvedAt)) {