Well, just thinking out loud, you could book the UA-only version of your itinerary, assume it tickets within 30 min, then apply the upgrade, then call and make your change to include your actual European destination/segments and ask for the change fee to be waived because it's within 24 hours of purchase. If that doesn't work, you could cancel and refund the itinerary (for free because it's within 24 hours of purchase.)