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.
modules/data-streams/src/main/java/org/elasticsearch/datastreams/DataStreamIndexSettingsProvider.java
private List<String> findRoutingPaths(String indexName, Settings allSettings, List<CompressedXContent> combinedTemplateMappings) {
        var tmpIndexMetadata = IndexMetadata.builder(indexName);
        int dummyPartitionSize = IndexMetadata.INDEX_ROUTING_PARTITION_SIZE_SETTING.get(allSettings);
        int dummyShards = allSettings.getAsInt(
            IndexMetadata.SETTING_NUMBER_OF_SHARDS,
            dummyPartitionSize == 1 ? 1 : dummyPartitionSize + 1
        );
        int shardReplicas = allSettings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0);
        var finalResolvedSettings = Settings.builder()
            .put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT)
            .put(allSettings)
            .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, dummyShards)
            .put(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, shardReplicas)
            .put(IndexMetadata.SETTING_INDEX_UUID, UUIDs.randomBase64UUID())
            
            .put(IndexSettings.MODE.getKey(), IndexMode.STANDARD)
            .build();
        tmpIndexMetadata.settings(finalResolvedSettings);
        
        try (var mapperService = mapperServiceFactory.apply(tmpIndexMetadata.build())) {
            for (var mapping : combinedTemplateMappings) {
                mapperService.merge(MapperService.SINGLE_MAPPING_NAME, mapping, MapperService.MergeReason.INDEX_TEMPLATE);
            }
            List<String> routingPaths = new ArrayList<>();
            for (var fieldMapper : mapperService.documentMapper().mappers().fieldMappers()) {
                extractPath(routingPaths, fieldMapper);
            }
            for (var template : mapperService.getAllDynamicTemplates()) {
                if (template.pathMatch() == null) {
                    continue;