Directory Server 5.1, 5.2 and 6.3 are compatible with regard to replication configuration, with the following exceptions:
Replication priority is not supported in releases prior to Directory Server 6.3. If you configure replication priority on a 6.3 master replica, the replication priority will be transferred to consumers running Directory Server 6.3, but not to any consumers running a previous version of Directory Server.
An unlimited number of masters is not supported on replication topologies that contain Directory Server 5.1 or 5.2 masters. Although Directory Server 6.3 supports an unlimited number of masters in a replication topology, this number is limited to four if your replication topology includes any Directory Server 5.2 master servers. Directory Server 5.1 does not support multimaster replication.