Also to be perfectly clear that "first registered first served" rule applies to all registrations -- if it were not registered by someone else already, I could register N123UA or N1234U for my (hypothetical) personal Cessna just as easily as I could register N44503. United certainly can "reserve" N-numbers -- but IIRC that's the same cost and paperwork burden as actually registering an aircraft. UA doesn't just get to claim anything ending with UA (wandering off topic I always wondered why a large number of NW aircraft were registered as NxxxUS... And Delta is just...I guess their mantra is "if it already has an N-number there's no reason to change it"