AD user export, modify and then import users

Hi

We are using Windows server 2008 R2 domain controllers, we like to export all AD users and modify / add phone & fax numbers. We tried below command to export the details

ldifde -f Exportuser.ldf -s  -d "ou=Test OU,dc=mydomain,dc=com" -p subtree -r "(&(objectCategory=person)(objectClass=User)(givenname=*))" -l "cn,givenName,objectclass,samAccountName,facsimileTelephoneNumber,mobile,pager"

After modifying the property we are unable to import them using the -i switch, giving error "Already Exists"

So how can we import the modified users either using ldifde / using powershell

Thanks in Advance

LMS