Christian,
There is an alert warning for MDB users who have multival-hi and multival-lo configured. If you using these then a database reload needs to be part of your upgrade plan. Otherwise, take one master down, perform a backup using slapcat, update the software and bring it back up. Wait for replication to catch up. Then repeat the process on the second master. You should be able to avoid downtime and dataloss.
If you're going to the trouble to upgrade, why not use 2.4.46 which has several improvements and fixes over 2.4.45. Just a thought.
Warmest regards, Jason Trupp Symas Corporation (855) LDAP-GUY