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
}
        if (name.toLowerCase(Locale.ROOT).equals(name) == false) {
            validationErrors.add("name must be lower cased");
        }
        for (String indexPattern : indexPatterns) {
            if (indexPattern.contains(" ")) {
                validationErrors.add("index_patterns [" + indexPattern + "] must not contain a space");
            }
            if (indexPattern.contains(",")) {
                validationErrors.add("index_pattern [" + indexPattern + "] must not contain a ','");
            }
            if (indexPattern.contains("#")) {
                validationErrors.add("index_pattern [" + indexPattern + "] must not contain a '#'");
            }
            if (indexPattern.contains(":")) {
                validationErrors.add("index_pattern [" + indexPattern + "] must not contain a ':'");
            }
            if (indexPattern.startsWith("_")) {
                validationErrors.add("index_pattern [" + indexPattern + "] must not start with '_'");
            }
            if (Strings.validFileNameExcludingAstrix(indexPattern) == false) {
                validationErrors.add(
                    "index_pattern [" + indexPattern + "] must not contain the following characters " + Strings.INVALID_FILENAME_CHARS
                );
            }
        }
        if (settings != null) {
            try {
                
                indexScopedSettings.validate(settings, true);