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/MetadataIndexTemplateServiceTests.java
Collections.singletonList("logs-*-*"),
            new Template(
                builder().put("index.mode", "time_series").put("index.routing_path", "uid").build(),
                new CompressedXContent(generateTsdbMapping()),
                null
            ),
            null,
            100L,
            null,
            null,
            new ComposableIndexTemplate.DataStreamTemplate(false, false),
            null
        );
        state = service.addIndexTemplateV2(state, false, "logs", template);
        var now = Instant.now();
        var mBuilder = Metadata.builder(state.getMetadata());
        DataStreamTestHelper.getClusterStateWithDataStream(
            mBuilder,
            "unreferenced",
            List.of(Tuple.tuple(now.minus(2, ChronoUnit.HOURS), now))
        );
        DataStreamTestHelper.getClusterStateWithDataStream(
            mBuilder,
            "logs-mysql-default",
            List.of(Tuple.tuple(now.minus(2, ChronoUnit.HOURS), now))
        );
        var stateWithDS = ClusterState.builder(state).metadata(mBuilder).build();
        var e = expectThrows(IllegalArgumentException.class, () -> {
            ComposableIndexTemplate nonDSTemplate = new ComposableIndexTemplate(
                Collections.singletonList("logs-*-*"),