Airlines paint planes all at the same rate of renewal. It has to do with the inspection process. Every _____ years the plane must be stripped to it's bare metal, inspected and is then repainted. Or at least this is what a seat mate who sells paint stripper to the airlines (seriously that was his job) told me. With a merger I personally think it's wasteful (environmentally, and bottom line cost) to rush this before it's needed as part of the inspection process. DL was very eager to paint the NWA 747's in its livery (though the inspection may have been do soon anyways).