DNS propagation and TTLs

"Propagation" is a misleading word. Nothing is pushed anywhere; each resolver simply keeps the old answer until its cached copy expires. How long that takes is the TTL of the record you changed, counted from the moment each resolver last fetched it.

Planning a change

1. A day or two before, lower the TTL of the records you will change to 300 seconds. Existing caches still hold the old TTL, so you need to wait out the old value once.
2. Make the change.
3. Check from several resolvers. Within five minutes everyone sees the new record.
4. Raise the TTL back to its normal value.

Changing nameservers

Nameserver changes are slower because the NS records at the parent zone (for .co.uk, Nominet's servers) typically have a TTL of a day or two. Keep the old nameservers running with the same zone until the parent TTL has expired everywhere.

Negative caching

A resolver that received "no such record" caches that too, for the time in the SOA minimum field. If you query a name before creating it, you may have to wait for that negative answer to expire.

Checking yourself

The DNS facet lets you query a specific resolver by adding @1.1.1.1 after the name in the Lookup bar, and shows each authoritative server's serial so you can tell whether your own servers have all picked up the change before blaming caches elsewhere.