I have domain.com, and it's currently served by ns1/ns2.oldisp.com. I need to change the authoritative nameservers to ns1/ns2.newisp.com.

Please tell me if my understanding of this is correct:

* Every domain has NS records which are authoritative for the domain in question. 

domain.com. IN NS ns1.oldisp.com.
* Every domain also has an authoritative nameserver that can be set via the registrar. My registrar says: "Changes to your name servers are sent to the registry immediately. However, it takes 24-48 hours for the changes to fully take effect."
* When a client asks for my domain, it will ask the root servers for the authoritative domain server for .com. That will give them a.gtld-servers.net (for example), and then it will ask that server for the authoritative server for domain.com, e.g. ns1.oldisp.com. That's provided, and away we go....

I want the switch to happen quickly, so how do I tell the authoritative server for .com (e.g. a.gtld-servers.net ) to lower the TTL on my domain.com record, so clients don't cache which nameservers are authoritative (and thus keep ns1.oldisp.com around for 24-48 hours)? Is that even possible?