You don't own any particular aircraft that may have been assigned to your flight days before departure. If UA(X) swaps, they swap. If your flight has no airplane because the airplane now assigned to your flight doesn't make it in (or there's no plane assigned to your flight) due to weather, your flight is cancelled due to weather. It doesn't help UA's cancellation stats any, since 5910 would be cancelled for the same reason if they didn't swap.