I'm trying to make a seamless transition from Heroku to AWS. Currently, DNS is being handled by Route53, with a CNAME record sending traffic to the heroku SSL subdomain. I need to change this to an A Alias record, pointing to the new Elastic Load Balancer with no service disruption.
If the initial record were an A record, then it would simply be a matter of creating the Alias A record, and then removing the original, so that there would always be an active A record for the given subdomain at all times. However, CNAMEs prevent any other records from being created for the same subdomain, with the following message:
<Error><Type>Sender</Type> <Code>InvalidChangeBatch</Code> <Message>RRSet of type A with DNS name www.example.com. is not permitted because a conflicting RRSet of type CNAME with the same DNS name already exists in zone example.com.</Message></Error>
If I remove the CNAME record before creating the A record, then there will be a window during which expired TTLs will fail to resolve the site. How can I avoid this downtime?