I am sending this as the body. it returns HTTP 204, but does not change the owner to me. The account stays attached to the current owner. I do have permission to change the owner and I can change simple parameters like address.
<?xml version="1.0" encoding="UTF-8"?>
<entry xmlns:d="http://schemas.microsoft.com/ado/2007/08/dataservices"
xmlns:m="http://schemas.microsoft.com/ado/2007/08/dataservices/metadata"
xmlns="http://www.w3.org/2005/Atom">
<m:properties>
<d:OwnerId >
<d:Id >3d967a88-dfd1-e111-bad5-73e3b508f847</d:Id>
<d:LogicalName>systemuser</d:LogicalName>
<d:Name>David Wright</d:Name>
</d:OwnerId>
</m:properties>
</content>
</entry>