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
+ " from existing composable templates [%s] with patterns (%s); this template [%s] may be ignored in favor"
                        + " of a composable template at index creation time",
                    request.name,
                    request.indexPatterns,
                    Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                    overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(",")),
                    request.name
                );
                logger.warn(warning);
                HeaderWarning.addWarning(warning);
            } else {
                
                String error = String.format(
                    Locale.ROOT,
                    "legacy template [%s] has index patterns %s matching patterns"
                        + " from existing composable templates [%s] with patterns (%s), use composable templates"
                        + " (/_index_template) instead",
                    request.name,
                    request.indexPatterns,
                    Strings.collectionToCommaDelimitedString(overlaps.keySet()),
                    overlaps.entrySet().stream().map(e -> e.getKey() + " => " + e.getValue()).collect(Collectors.joining(","))
                );
                logger.error(error);
                throw new IllegalArgumentException(error);
            }
        }
        templateBuilder.order(request.order);
        templateBuilder.version(request.version);
        templateBuilder.patterns(request.indexPatterns);
        templateBuilder.settings(request.settings);