I purchased economy plus upgrades on an upcoming flight. United changed the aircraft on this flight and when I checked the seating chart I found our assigned seats were no longer in economy plus. I received no notification from United regarding this change so called and was told that they could not change our seats to economy plus as the cost for economy plus was greater on this aircraft than our original reservation. Should I want economy plus I would need to pay an additional $60. I would have thought once a reservation was made and paid for United could not change the price due to an equipment change on their end. Is this normal?