If you look at the `RESTSerializer`'s code for `serializePolymorphicType` ([source](https://github.com/emberjs/data/blob/master/packages/ember-data/lib/serializers/rest_serializer.js#L780-L786)) it uses the CamelCase form.

I think the example in the [transition guide](https://github.com/emberjs/data/blob/master/TRANSITION.md#polymorphic-relationships) shows how to use the underscored form:

```javascript
serializePolymorphicType: function(record, json, relationship) {
    var key = relationship.key,
        belongsTo = get(record, key);
    key = this.keyForAttribute ? this.keyForAttribute(key) : key;
    json[key + "_type"] = belongsTo.constructor.typeKey;
  }
```
Copying that should get it working with underscores.