How to replace account with inetOrgPerson?

[Mi]

I am trying to add the "inetOrgPerson" objectClass, but some users already have the 
"account" objectClass.

After a long search, I found that you cannot have both. So I am trying to remove 
"account", and add "inetOrgPerson". But I cannot do that either. I just get the 
following error :

    err=69 text=structural object class modification from 'account' to 
'inetOrgPerson' not allowed

If I just try to remove "account", I get

    entry failed schema check: no structural object class provided

So, how can I add "inetOrgPerson" and remove "account" ?

Also, I notice that I don't have "account" listed under dn: 
cn=schema,cn=config. What I have is:

  {0}core
  {1}cosine
  {2}nis
  {3}inetorgperson
  {4}samba
  {5}apple


Thanks for any help

mi

PS: I just need a "mail" attribute. Maybe there is a simpler way than to add inetOrgPerson with all 
it's ridiculous attributes like "audio" or "carLicense" ?