This early it should be fairly easy to confirm an upgrade using mileage. If this is the only segment of your trip on
UA, try calling Mileage Plus directly to ask them to upgrade this portion. Do not wait until check-in to try and upgrade. If you have the miles now, and want to use them, then you should confirm your upgrade ASAP. You are on the first morning flight out to
JFK which is usually popular, depending on the day of the week.