2.4.42 is a bug fix release for significant issues that were in 2.4.41. No new features were added. Primarily for bugs in slapd-mdb and the lmdb library itself. I would upgrade to 2.4.42.
As for your question about different versions there is no issue replicating between 2.4.39 and 2.4.41 or .42
I would note that Howard checked in some potential syncrepl optimizations for slapd-mdb into the master branch this week that may help with your replication delays if you want to test them on top of 2.4.42.
On Aug 15, 2015, at 6:58 AM, Brian Wright <email@example.com> wrote: