Seat Guru (http://www.seatguru.com) is your friend. You can enter an airline, flight number and date and it will show you the layout and give you specific seat reviews. E.g., window misaligned, seat restricted recline, close to galley/toilets, etc.
Airlines switch out equipment on routes all the time, and not all planes have the same layout. When I booked my (sadly cancelled) flights to Italy last December the SFO/FRA legs were on a United 777-300ER with (true) Polaris Business and Premium Economy, Economy Plus (extra legroom), and Standard Economy seats. I had picked seats in Polaris Business (1-2-1 across). Early spring the plane was swapped to a 787-9 with the old layout, business seats were the old style (narrower 2-2-2 across) and no Premium Economy. Mid-summer it switched back to the 777-300ER as demand improved. (United claims its remaining old layout 787-9s will be re-configured by this summer).
Unfortunately if they switch the plane they may re-map your seat selection but you may not get what you thought you had when you originally booked. As others have noted above with Delta, this is not limited to United. When the equipment was switched out, my seats moved, seemingly at random. The only thing you can do is check your booking every week-- most airlines post their flight changes on Saturday, so check every Sunday or Monday morning. If you paid for a Premium seat the the equipement change eliminated that class you should be due a refund for the fare difference.