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
final String name;
        final String cause;
        boolean create;
        int order;
        Integer version;
        List<String> indexPatterns;
        Settings settings = Settings.EMPTY;
        CompressedXContent mappings = null;
        List<Alias> aliases = new ArrayList<>();
        TimeValue masterTimeout = MasterNodeRequest.DEFAULT_MASTER_NODE_TIMEOUT;
        public PutRequest(String cause, String name) {
            this.cause = cause;
            this.name = name;
        }
        public PutRequest order(int order) {
            this.order = order;
            return this;
        }
        public PutRequest patterns(List<String> indexPatterns) {
            this.indexPatterns = indexPatterns;
            return this;
        }
        public PutRequest create(boolean create) {
            this.create = create;
            return this;
        }
        public PutRequest settings(Settings settings) {
            this.settings = settings;
            return this;
        }