In theory the existing migration script will transfer private DNS:

• The eqiad1-r instance gets a proper DNS entry
• During migration there are duplicate entries (foo.bar.wmflabs.org points to two different IPs)
• Post-migration the initial VM is deleted at which point the old DNS entry is automatically cleaned up

That's probably fine, although it needs more testing.

Public DNS with floating IPs needs more work. We should be able to identify all floating IPs associated with a given VM, but finding all DNS records that point to that IP might not be straightforward.