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
validateTimestampFieldMapping(mapperService.mappingLookup());
                }
            } catch (Exception e) {
                throw new IllegalArgumentException("invalid composite mappings for [" + templateName + "]", e);
            }
            return null;
        });
    }
    private static void validateTemplate(Settings validateSettings, CompressedXContent mappings, IndicesService indicesService)
        throws Exception {
        
        Settings settings = validateSettings;
        if (settings == null) {
            settings = Settings.EMPTY;
        }
        Index createdIndex = null;
        final String temporaryIndexName = UUIDs.randomBase64UUID();
        try {
            
            int dummyPartitionSize = IndexMetadata.INDEX_ROUTING_PARTITION_SIZE_SETTING.get(settings);
            int dummyShards = settings.getAsInt(
                IndexMetadata.SETTING_NUMBER_OF_SHARDS,
                dummyPartitionSize == 1 ? 1 : dummyPartitionSize + 1
            );
            int shardReplicas = settings.getAsInt(IndexMetadata.SETTING_NUMBER_OF_REPLICAS, 0);
            
            Settings dummySettings = Settings.builder()
                .put(IndexMetadata.SETTING_VERSION_CREATED, Version.CURRENT)
                .put(settings)
                .put(IndexMetadata.SETTING_NUMBER_OF_SHARDS, dummyShards)