You cannot migrate using replication, ubuntu 7.1 comes with openldap 2.2 which dont have syncrepl, It only have slurpd. Ubuntu 11 comes with openldap 2.4 which have syncrepl not slurpd. It is possible to migrate online from Openldap 2.2 to Openldap 2.3 & openldap 2.3 to Openldap 2.4, but definitely not Openldap 2.2 to Openldap 2.4.
You need to take backup of your ldap tree & add again.
Take the back up data on old LDAP server system using ldapsearch or slapcat
$ldapsearch -x -LLL -b "dc=yourbase,dc=com" -D "cn=Manager,dc=yourbase,dc=com" -W >> mybackup.ldif
or use slapcat, but make sure slapd is stopped before running slapcat.
$slapcat -l /tmp/backup.ldif
After exporting it to a file,Remove the below attributes from the backup file. If slapcat method is used.
Import the ldif file using ldapadd command
$slapadd -F /etc/openldap/slapd.d/ -n 0 -l /tmp/backup1.ldif
For cases where third party schema files are being used in ubuntu 7.1 , after copying the schema files to ubuntu11, if any schema violation occurs like OID's used in third party schema files may not be compatible with OpenLDAP versions of ubuntu 11, In which case contact the vendor who have provided the schema files.