When you go to delete the Primary NIC of an instance, you are given an error if there are other (secondary) NICs attached:

It would be nice to improve this workflow a bit so a user does the right thing without encountering an error to learn what the right thing to do is.
@internet-diglett had the idea to gray out the Delete button on the Primary NIC and have a hover message to the effect of "Cannot delete the Primary NIC while other NICs are present. You must promote another NIC to Primary before deleting this one."