The OpenLDAP on-disk format has changed between the 1.2.x and 2.0.x releases.

You will very likely need to dump out your directory to a text file using
"ldbmcat -n" before upgrading and re-import the entries into the new format
using "slapadd" after the upgrade.

Because schema checking is now available and enabled by default, some entries
may not import cleanly.  In particular, if you have used previous versions of
the migration scripts, some of the entries (particularly oncRpc objects) may
be missing "description" attributes which are now required by the schema.

If this is the case, you will need to either edit the LDIF files prior to
importing them, or add "schemacheck off" to your new slapd.conf file to disable
schema checking while the entries are being imported.  (Disabling schema
checking on your server is strongly discouraged, however.)