After a complex set of steps to setup ipv6 in my [VPC, Router, Gateway, Security Groups, and others that I'm forgetting], I was able to launch 2 instances and get both ipv4 and ipv6 addresses. As OddBloke notes, I did have to 'sudo dhclient -6' to grab that ipv6 address in the instance. Once I did, I was able to ssh back and forth using ipv6 (yay!). Then I tried 'apt update'. In fact, the mirrors inside of AWS worked us fine. But security.ubuntu.com did not. It did resolve, but it didn't respond. Rest assured we (Canonical) are working this right now... Thanks.