NEED TO CHANGE FILR LDAP CN TO UID

Hi

Currently I am running FILR (on Hyper V) syncing with LDAP using AD

When I set it up I used the CN in LDAP attribute used for Filr name:
I need to change this to UID

I have tried to change it to UID, and when i redo the LDAP sync, all my users are blanked out (as if they have been deleted)

When I change it back to CN, they reappear

Anyone know how I can get get this to work?

Thanks in advance