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
template.indexPatterns(),
                finalTemplate,
                template.composedOf(),
                template.priority(),
                template.version(),
                template.metadata(),
                template.getDataStreamTemplate(),
                template.getAllowAutoCreate()
            );
        }
        if (finalIndexTemplate.equals(existing)) {
            return currentState;
        }
        validateIndexTemplateV2(name, finalIndexTemplate, currentState);
        logger.info(
            "{} index template [{}] for index patterns {}",
            existing == null ? "adding" : "updating",
            name,
            template.indexPatterns()
        );
        return ClusterState.builder(currentState).metadata(Metadata.builder(currentState.metadata()).put(name, finalIndexTemplate)).build();
    }
    /**
     * Calculates the conflicting v2 index template overlaps for a given composable index template. Optionally if validate is true
     * we throw an {@link IllegalArgumentException} with information about the conflicting templates.
     * <p>
     * This method doesn't check for conflicting overlaps with v1 templates.
     * @param currentState the current cluster state
     * @param name the composable index template name
     * @param template the full composable index template object we check for overlaps