thanks, as stated in my other e-mails this was the answer I was looking
for, I didn't know about this (and didn't think of it).
Note that when changing between minor versions it is usually required to
slapcat/slapadd the database, as binary compatibility is not guaranteed.
In the case of OpenLDAP 2.3/2.4, as far as I recall, no database changes
occurred. However, there were minor changes (significantly, support for
indexing on more matching rules for more syntaxes) which required to
rebuild the indexes.