objectclass question

[Michal Gubik <mgubik@wolfhill.cz>]

My question is regarding object class. I have made already several users 
on ldap but I need to add host atribute that is in account objectclass 
but when I tri to add account objectclass to that user I get error 
saying ldap_modify: Cannot modify object class (69)
       additional info: Structural object class modification from 
'InetOrgPerson' to 'account' not allowed
my ldif looks like this:

dn: uid=mgubik, ou=people, o=coprosys,dc=cd
changetype: add
objectclass: account
add: host
host: hostname

and  I try to add it by ldapmodify.  Thanks for any help.