Why the TTL has to be lowered beforehand
The TTL decides how long resolvers cache a DNS record. At 24 hours the old IP can still be served for up to a day after a change. So the TTL is lowered at least as long before the move as the old TTL — only then do all caches hold the low value.
Typical pitfalls
- NS delegation: the TTL of the name-server records at the registry (often 24 h or more) cannot be lowered by you. When switching name servers, old and new servers must answer identically.
- Minimum TTL: some resolvers enforce a floor; “300 seconds” is not exact everywhere.
- Negative caching: missing records are cached according to the SOA minimum / SOA TTL.
- Old environment: keep it running for 48–72 hours; migrate mailboxes separately.