United Airlines in Sydney

[Kieran Wells]

After spotting last wednesday, i noticed something interesting with the two United Airlines flights.
UA840 which came in from Melbourne, left Sydney as the flight heading for SFO and ORD as UA870. This plane was in the 'old' silver united livery. Whilst the plane that did leave as UA840 for LAX was the 'new' blue livery.

Can anyone shed some light on this change of planes in Sydney for UA840, or was it the simple fact that the flight from melbourne was running late(40mins due to the wind storm),and planes were just swapped?