You can change all the CUCM directory entries from LDAP if if was synched. Not the actual Directory numbers assigned to the phone. The LDAP Directory synchronization is for directory lookups and presence information. It does not directly control the extension numbers (DNs) of the IP Phones.
I agree with Kasey. The LDAP phone number that is synchronized is only the number that appears in the Corporate Directory. The field it is synchronized can vary as ours does using AD. In our case, the Phone number is what Jabber uses, and the SIP number is what is synchronized with the Corporate Directory. This does not effect the DN (Directory Number) assigned to the phone.