Originally Posted by stevenou

I did this mid-Dec 17 and had no issues. As mentioned above, they simply split off to different reservation and cancel the new one. This was a United award operated by a partner booked directly on United.com.

I've had complications in the past when the partner locks United out of making changes (happened with SWISS). It's still possible to get it done but it may take escalating to a more experienced agent plus they need to get in touch with the partner airline so you could be on hold for a while.