How to sync ldap attribute for users between two environments?

We are in the proccess to upgrade to 901 from 900 environment. At the initial 901 build we migrated all users from 900 to 901 a few months. During the past months quite a few users ldap attribute have been changed and new users have been added to 900 environments. With that said, two environments users are out of sync. Is there a way so that I can sync 901 with 900?