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 (dataStream.getIndexMode() != IndexMode.TIME_SERIES) {
                continue;
            }
            if (updatedMetadata == null) {
                updatedMetadata = Metadata.builder(state.metadata()).put(templateName, newTemplate).build();
            }
            var matchingTemplate = findV2Template(updatedMetadata, dataStream.getName(), false);
            if (templateName.equals(matchingTemplate)) {
                if (currentMetadata.isTimeSeriesTemplate(newTemplate) == false) {
                    if (dataStreamsWithNonTsdbTemplate == null) {
                        dataStreamsWithNonTsdbTemplate = new HashSet<>();
                    }
                    dataStreamsWithNonTsdbTemplate.add(dataStream.getName());
                }
            }
        }
        if (dataStreamsWithNonTsdbTemplate != null) {
            var settings = MetadataIndexTemplateService.resolveSettings(newTemplate, currentMetadata.componentTemplates());
            var routingPaths = IndexMetadata.INDEX_ROUTING_PATH.get(settings);
            throw new IllegalArgumentException(
                "composable template ["
                    + templateName
                    + "] with index patterns "
                    + newTemplate.indexPatterns()
                    + ", priority ["
                    + newTemplate.priority()
                    + "]"
                    + ", index.routing_path "
                    + routingPaths
                    + " "