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
}
                    missingNames.add(name);
                }
            }
            if (missingNames != null) {
                throw new IndexTemplateMissingException(String.join(",", missingNames));
            }
        } else {
            final String name = names[0];
            for (String templateName : currentState.metadata().templatesV2().keySet()) {
                if (Regex.simpleMatch(name, templateName)) {
                    templateNames.add(templateName);
                }
            }
            if (templateNames.isEmpty()) {
                
                
                boolean isMatchAll = false;
                if (Regex.isMatchAllPattern(name)) {
                    isMatchAll = true;
                }
                if (isMatchAll) {
                    return currentState;
                } else {
                    throw new IndexTemplateMissingException(name);
                }
            }
        }
        Set<String> dataStreamsUsingTemplates = dataStreamsUsingTemplates(currentState, templateNames);
        if (dataStreamsUsingTemplates.size() > 0) {