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
public PutRequest mappings(CompressedXContent mappings) {
            this.mappings = mappings;
            return this;
        }
        public PutRequest aliases(Set<Alias> aliases) {
            this.aliases.addAll(aliases);
            return this;
        }
        public PutRequest masterTimeout(TimeValue masterTimeout) {
            this.masterTimeout = masterTimeout;
            return this;
        }
        public PutRequest version(Integer version) {
            this.version = version;
            return this;
        }
    }
    public static class RemoveRequest {
        final String name;
        TimeValue masterTimeout = MasterNodeRequest.DEFAULT_MASTER_NODE_TIMEOUT;
        public RemoveRequest(String name) {
            this.name = name;
        }
        public RemoveRequest masterTimeout(TimeValue masterTimeout) {
            this.masterTimeout = masterTimeout;
            return this;
        }
    }
}