I have a RHEL 6 OpenLDAP 2.4.40 publisher with three subscribers in production of the same version. In development i also have a RHEL 6 OpenLDAP 2.4.40 and one subscriber of the same version.

I got a RHEL 7 OpenLDAP 2.4.44 subscriber replicating in development.
I was able to load via slapadd the subscriber in about 30 minutes from an overnight slapcat ldif.files from the publisher.

How can I upgrade the publisher without too much down time and ultimately do the same in production? I want to also keep the same host names.