How to replace account with inetOrgPerson?

[mi]

I am trying to add the "inetOrgPerson" objectClass, but the 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 ?

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" ?