Just went through the process to do this...for an upgrade to First. If ticket is booked on United stock with UA flight numbers, you need to call an agent to have them convert the flights to pure NH flight numbers before processing the upgrade request online. The online process is seamless, but the process of switching the flights over to NH-coded flights was a bit of a cluster. NH kept the UA flight number on the reservation and it took a few iterations for them to get rid of it so that there were just the NH flights there. Make sure to give yourself plenty of time to stay on the phone with UA if you want to get this done...