Change structural objectClass

Hi! I have an ldap installation with almost 100 users in it, and now I've realised I need to change the structural objectClass. Because I need some attributes I don't have and my current objectClass won't allow me to have this attributes (or that's what I think). Now my objects are accounts and I need them to be organizationalPerson, so I can have telephon info, and picture, and some other things. Is there a way I can make this change without having to create all the users again? It's not that much for the time I have to waste but just because I don't know the users passwords so I would have to ask them to enter their password again and I think they would kill me if I asked them to do so. So if you know a way to just create new accounts and then import the passwords from the old ones that'll work for me too. Thanks a lot!