Sign into your account. Go to the "My trips" tab. At the bottom, you'll see "find another trip". Enter the confirmation # and add it to your trips.

United has premium economy (PremiumPlus is the actual wording) seats (separate cabin from economy, sold as O, A, R fares), economy plus seats (economy seats w/ a little extra legroom), and economy seats. Usually a premium economy (PremiumPlus) fare purchasers won't find themselves in coach after an aircraft swap. If you don't have a O/A/R fare, you do not have premium economy seats. Are you sure you don't actually have Economy Plus seats?