I heard (perhaps wrongly) that using an Amazon Elastic IP "EIP" on a network interface and attaching that to an EC2 instance might be a quicker way of bringing up (or switching to) a new machine.
However I am little confused how to handle the networking once the new eth1 is attached. Do people have a eth1 systemd networkd rule with a low RouteMetric or do they remove eth0, or they just alter the route or am I missing a trick?
Ultimately since ELB can't have a static IP address, I am looking for ways of making sure EC2 can be quickly provisioned to avoid down time.
You can just assign the Elastic IP to the existing interface. Unless you have a compelling reason to keep the original public IP address associated with the instance, there's probably no reason to use a second interface. No change on the instance itself is needed when assigning an Elastic IP to the instance on an existing interface, as traffic for public IPs in AWS is all handled by NAT to the private IP address.
http://docs.aws.amazon.com/AWSEC2/latest/UserGuide/using-instance-addressing.html
If you manage your zone with Route53, though, you can use their "Alias" record type for an ELB. It's not the same as a static IP, but it suffices in many use cases.
http://docs.aws.amazon.com/Route53/latest/DeveloperGuide/resource-record-sets-choosing-alias-non-alias.html