Example: example.com is registered at Namecheap but I want to use Cloudflare DNS. So on my Namecheap DNS I set the domain's nameservers (I thought this changed the NS records) to the Cloudflare name servers - deb.ns.cloudflare.com and sri.ns.cloudflare.com). Later, then I do an NSLOOKUP directly again namecheap's name servers (dns1.registrar-servers.com) and query the NS records, I expected to see deb.ns.cloudflare.com and sri.ns.cloudflare.com. But I do not. I see namecheap's nameservers showing.
nslookup - dns1.registrar-servers.com
> set type=NS
> example.com
Server: dns1.namecheaphosting.com
Address: 216.87.155.33
example.com nameserver = dns1.registrar-servers.com
example.com nameserver = dns2.registrar-servers.com