Modifying the queue block size is a major change to the Replication Server configuration and affects all connections to the Replication Server. You must suspend log transfer and quiesce Replication Server.
In the queue block size change procedure, "upstream" refers to all replication system components that feed data to the Replication Server where you want to change the queue block size and "downstream" refers to the components that receive data from the affected Replication Server.