Adding to Kuri's statement, editing a preset's images and/or codes will not effect the already exported preset. When you re-export a preset after updating or making fixes to it, what have you, the game will automatically overwrite the existing exported preset. This means, that whatever character is wearing the preset, will be updated to the new one on its own after the preset's been re-exported. And as Kuri said, those that have your previous preset will continue to see the old one until they download the new one. The only time a preset will appear black and yellow is when the image is missing.