changing 'cn'

[Anil]

If I have an entry like this:

dn: cn=unique-abc, ou=people, o=isp
objectclass: top
...
cn: unique-abc
cn: hello

or

dn: cn=unique-abc, ou=people, o=isp
objectclass: top
...
cn: unique-abc
cn: hello
cn: people

How do I go about changing 'hello' to 'world' using modify() (or some 
other method)?

It seems there are only ADD/DEL/REPLACE operations, which don't work in 
this environment. Is it possible to pass a modlist that replaces only 
the hello instead of replacing everything?

(basically, I want to keep the RDN intact, instead just be able to make 
changes to non RDN values)