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
List.of(),
                twoHoursAgo.toEpochMilli(),
                builder().build(),
                1
            ).getMetadata()
        );
        DataStream ds = mb.dataStream(dataStreamName);
        mb.put(
            new DataStream(
                ds.getName(),
                ds.getIndices(),
                ds.getGeneration(),
                ds.getMetadata(),
                ds.isHidden(),
                ds.isReplicated(),
                ds.isSystem(),
                ds.isAllowCustomRouting(),
                IndexMode.TIME_SERIES
            )
        );
        Metadata metadata = mb.build();
        Instant now = twoHoursAgo.plus(2, ChronoUnit.HOURS);
        Settings settings = Settings.EMPTY;
        Exception e = expectThrows(
            IllegalStateException.class,
            () -> provider.getAdditionalIndexSettings(
                DataStream.getDefaultBackingIndexName(dataStreamName, 1),
                dataStreamName,
                true,
                metadata,