2.3 -> 2.4 upgrading advice

[Rex Roof <rex@wccnet.edu>]

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.