anonymous DNS

[CK]

Currently I have a few domains registered on our servers.  When we
registered them, we inputted dns1.ourserver.com and dns2.ourserver.com
for the primary and secondary.  What would I have to do to change this.
I want their WHOIS output to be:

dns1.theirserver.com
dns2.theirserver.com

I only have 2 servers doing dns for us.  They are,

dns1.ourserver.com
dns2.ourserver.com

If I add another ip address to the hostname we currently use and than
add an alias to that ip, would that work?  I am not sure and I have a
client who is quite curious about this matter.  Thanks for your
assistance, and again, sorry to bother you on vacation.

Chris