2.3 -> 2.4 upgrading device

[Rex Roof]

I'm currently running a fairly simple LDAP environment of two OpenLDAP
2.3 servers with a Master & Slave relationship using syncrepl. These
servers are currently being used for authentication and for sendmail
routing.

I'm hoping to upgrade these two OpenLDAP 2.4 and create some sort of
multi-master environment so that I can accept writes when one or more
of the servers is down. I'd also like to add 2 more servers to the
pool.

I have two questions:

1) does anyone have any advice for things to watch out for when
updating from 2.3 to 2.4?
2) what should I use for replication? Can I use some combination of
MirrorMode and Multi-Master replication? This statement worries
me: "Breaks the data consistency guarantees of the directory model"
what does it mean? I found that here:
http://www.openldap.org/doc/admin24/replication.html

is that only a concern when the servers get out of sync as far as time
goes?

Any advice would be appreciated.