You can verify/check using server_cifssupport against the new SECONDARY CIFS server:  $  server_cifssupport server_2 -secmap -create -name  -domain 
   
    where is a NEW user who has not yet been mapped to the Celerra.  If this succeeds without error, your Usermapper setup is working correctly, as it will query the primary for a new UID/GID pair.
Any tricks here to watch out for? Existing CX-480 as primary and plan to merge / convert to a secondary UM. Already have CIFS user/group files exported and ready to merge. Does CIFS have to be disabled or user activity stopped first?
UserMapper - Convert from Primary to Secondary