Bands should not be replaced except when the old band is causing injury or irritation to the bird or if the band is illegible.
If the old band is causing injury or irritation, it should be removed and a new band applied to the other leg. Bandit (the BBL banding data reporting program) has a field for reporting the replaced band number.
All removed bands should be sent to the BBL or BBO, preferably flattened and taped to cardboard with the replacing (new) number clearly identified next to it. If the band is illegible, it will be chemically etched to determine the band number. Do not report the bird as a recapture on the reportband.gov website.
Bird Banding Laboratory files will also be cross-referenced, thus maintaining the longevity for each bird.
SOME BIRDS ARE WEARING EXPERIMENTAL BANDS MADE OF VARIOUS METALS, WHICH ARE THINNER THAN THE STANDARD ALUMINUM BAND. DO NOT REMOVE THESE BANDS.