CUCM 11.x support both Local & LDAP Sync users simultaneously.
Once you enable LDAP intergation and if the userID (of Local User) matches an LDAP user, then the CUCM will convert that user into an LDAP user. Rest of the Local Users will remain as Local Users itself in CUCM.
Sometimes you have to wait for the overnight Garbage cleanup for the local account to be converted to LDAP User. If not, do the Full Sync.
All the information of the Local user (First Name, Last Name, Display Name, Telephone no., Mail ID, etc..) will be replaced as in the LDAP configuration. But all other configurations related to CUCM (Secutiy Group, Controlled devices, etc..) will remain same.
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Thanks,
Adnan