Network Solutions is hosting the DNS server for a domain I've just taken over. I want to move a host to a different IP. Sounds simple enough, I just change the A record and wait for the change to propagate (or set the TTL low and wait less time.) When I go to the web tool to change the IP I get this warning:
Warning, Service Interruption Will Occur!

In order to use Advanced DNS Manager, your Name Servers must be moved to Network Solutions managed name servers.

Please Note: When you Move Name Servers to Network Solutions, your Existing Name Servers will no longer be used. As a result, Web sites and E-mail from other providers will be lost.

Existing Name Servers

New Name Servers (## will be assigned upon clicking Go)

I don't understand the part about "Web sites and E-mail from other providers will be lost." If all they are doing is moving the domain to different name servers then why should anything be lost? I understand that if I change an A record or an MX record that different DNS servers could get different values for the length of the TTL and I could run into problems. But, just moving to a new name server should be safe, right?

What am I missing?

Btw, I'm posting here and not asking Network Solutions because they are terribly understaffed and their hold times are commonly an hour or longer.