Recommended way to transfer data from one cluster to the other?

[Karsten Heymann <karsten.heymann@gmail.com>]

Hi,

We're replacing our aging LDAP Cluster (Debian 7, slapd 2.4.40, 2
masters with syncrepl and mirror mode, 2 slaves) with a new cluster of
the same layout (2.4.44, probably 2.4.46, on Debian 9). I now wonder
what the best way is to transfer the data from the old to the new
cluster. As far as I can see, we have three options to transfer the
data:

1) ldapsearch + ldapadd
2) slapcat + slapadd
3) Configure a temporary syncrepl rule on the new masters to fetch the
data from the old ones.

Which one is the preferred one? I would assume slapcat and slapadd,
but I must admit I have not fully understood the purpose of the -S and
-w switch. Should we somehow filter the *CSN-fields from the exported
ldif as I assume they make no sense for the new cluster or are they
automatically fixed when using slapadd with said options?

I'm sorry if this topic has been discussed here before but my
(probably lacking) google skills did not return anything obvious.

Best regards
Karsten