HI, I duno about any tools but its very easy to migrate from idbm to back-bdb if you search the archives of few days back you will find a thread in which it was discussed, and i had successfully migrated our openldap to back-dbd.

here is the short summary of steps for this..

Migrate to BDB or HDB.  Here's the rough idea:
shutdown slapd
slapcat -f /path/to/slapd.conf -l mydb.ldif
vi /path/to/slapd.conf
 Change your database to a new directory.
 Read about tuning, cache sizing, and other stuff.
 Read openldap man pages.
 Read oracle's tuning docs for bdb.
 Re-Read openldap man pages.
:wq
slapadd -f /path/to/slapd.conf -l mydb.ldif

Fix permissions on /var/run so the slapd user can write there -- this
one is pretty easy
restart slapd