The corrode markers will not fall off. They only fall off if the creature's armor drops below zero. So for example, if you have Leather Gloves, Leather Boots, and Dragonscale Hauberk on your mage, and you have 3 corrode conditions, if you replace your Dragonscale Hauberk with another chest piece that gives armor +2, one of the corrode conditions would fall off because for the brief instant while you didn't have a chest piece, your armor dropped to -1.