Is it possible to migrate changes to OWD's / Sharing Rules via Changesets?
I know this is an old question, but either the answer has changed or the answer was wrong.
OWD is deployed as part of the metadata of the object.
https://developer.salesforce.com/docs/atlas.en-us.api_meta.meta/api_meta/customobject.htm
It looks like the answer is "no." Based on the components available listing, you can deploy sharing rules for individual objects but not the OWDs.