Although you don't want to override any managed fields when you update an instant campaign zip file - it may be that you've added new fields to be managed.
At the moment the fields to be managed are created on the initial creation of the MM component but if a client then opts to manage more fields they have to reupload the ZIP as a new component (thus removing the content changes previously made, losing previous versioning as the previous component is removed from the page a deleted) or manually tweak the embedded fields with the keys.
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