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.
server/src/main/java/org/elasticsearch/cluster/metadata/MetadataIndexTemplateService.java
}
        return overlaps;
    }
    private void validateIndexTemplateV2(String name, ComposableIndexTemplate indexTemplate, ClusterState currentState) {
        
        
        var finalTemplate = Optional.ofNullable(indexTemplate.template());
        var finalSettings = Settings.builder();
        final var now = Instant.now();
        final var metadata = currentState.getMetadata();
        final var combinedMappings = collectMappings(indexTemplate, metadata.componentTemplates(), "tmp_idx");
        final var combinedSettings = resolveSettings(indexTemplate, metadata.componentTemplates());
        
        for (var provider : indexSettingProviders) {
            finalSettings.put(
                provider.getAdditionalIndexSettings(
                    "validate-index-name",
                    indexTemplate.getDataStreamTemplate() != null ? "validate-data-stream-name" : null,
                    indexTemplate.getDataStreamTemplate() != null && metadata.isTimeSeriesTemplate(indexTemplate),
                    currentState.getMetadata(),
                    now,
                    combinedSettings,
                    combinedMappings
                )
            );
        }
        
        finalSettings.put(finalTemplate.map(Template::settings).orElse(Settings.EMPTY));
        var templateToValidate = new ComposableIndexTemplate(
            indexTemplate.indexPatterns(),