I had to migrated a few VMs from EC2 into a VPC and to do so, I created an image of every VM, stopped it, and launched a new instance in the VPC from the AMI I've created. I am now wondering, if I have to keep all the AMIs, once the new VMs are running. Is it safe to de-register them, or will this break the instances started with the respective AMI?
Launchable AMI's are independent of their 'child' instances, so can be safely deleted. Amazon doesn't do things like block-level deduplication of templates to children (at least, that they've admitted to) so there is no relationship between a source AMI and an instance other than an attribute in metadata.