Move your nameservers without touching the registrar
When your nameservers are hostnames you own, you can move every domain that uses them at once by changing two A records. How to do it safely.
Domains point at nameserver names
A domain is delegated to nameserver names such as ns1.example.com. The registrar stores those names. The addresses behind them live in the zone of example.com.
So when the old server is also your nameserver, you can move DNS for every domain at once. Change the A records of ns1 and ns2, and the registrars never need a visit.
Check for glue records first
If ns1.example.com is registered at the registry with its own address, that glue record wins for some resolvers. Change it at the registrar as well, or the move is only half done.
In our case the registrar held no glue for either name. Two A records in the zone were the only place the old address lived.
Load every zone before you point at the new server
The new nameserver must already answer for every domain delegated to those names. A zone it does not hold stops resolving the moment you switch.
List the zones on the old server, then ask a public resolver which ones really use these nameservers. On our old DirectAdmin box, 12 of its 40 zones did. We loaded those 12 from the current zone files.
Compare what both servers answer
Ask both servers the same questions and compare what they say. We asked for A, MX, TXT, NS, CAA and AAAA on each domain and its common hostnames.
That came to 648 answers, and none differed. Only then did we change the two A records.
After the switch
Keep the old nameserver running for a day. Some resolvers hold the old address for the length of its TTL, and both servers give the same answers meanwhile.
From then on, edit DNS only on the new server. Change a record on the old one and nobody will see it.
KLYRN can act as the nameserver for its own sites, or work through Cloudflare. The DNS guide covers both.