You should use the supported tools to migrate your database -- slapcat the master, then slapadd that to the replica... Same with rebuilding the master. That is the only officially supported upgrade path from 2.3 -> 2.4.
I did try that too. 'slapcat' took 6 minutes, and 'slapadd' to an empty directory wanted 15hours. If I move the ldif file to the x4500 where I have 16GB of /tmp, I can 'slapadd' it all in 13 minutes, then rsync the data back to the master and disk (3 minutes).
I just thought the whole rsyncing the binary db files to be 'uglier' than having 2.4 syncrepl against a 2.3 until it was up to date.
Lund -- Jorgen Lundman | <lundman@lundman.net> Unix Administrator | +81 (0)3 -5456-2687 ext 1017 (work) Shibuya-ku, Tokyo | +81 (0)90-5578-8500 (cell) Japan | +81 (0)3 -3375-1767 (home)