I have recently migrated one of my .com-domain zones from DNS provider A to DNS provider B, however the TTL on the NS-records for the zone on provider A is very high = 432000 (5 days), and thus I feel sad thinking that I might have to keep (and potentially update) the zone at the old provider for almost a weeks time.
Question is: Since WHOIS for the domain is already updated with the new nameservers of provider B, can I safely delete the zone at provider A, even though NS-record TTL might not have expired for clients?
Secondary question: What exactly happens when clients contact NS provider A and it doesn't respond with authority anymore? Provider A should just call Provider B and relay a non-authoritative response to the client, right?