1 files changed, 7 insertions, 0 deletions
diff --git a/develop/upgrade.html b/develop/upgrade.html
index f1ccf3ed49..49d7e7615e 100644
--- a/develop/upgrade.html
+++ b/develop/upgrade.html
@@ -242,6 +242,13 @@ working email server in the <code>email</code> configuration section (including,
 minimum, a <code>notif_from</code> setting.)</p>
 <p>Specifying an <code>email</code> setting under <code>account_threepid_delegates</code> will now cause
 an error at startup.</p>
+<h2 id="changes-to-the-event-replication-streams"><a class="header" href="#changes-to-the-event-replication-streams">Changes to the event replication streams</a></h2>
+<p>Synapse now includes a flag indicating if an event is an outlier when
+replicating it to other workers. This is a forwards- and backwards-incompatible
+change: v1.63 and workers cannot process events replicated by v1.64 workers, and
+vice versa.</p>
+<p>Once all workers are upgraded to v1.64 (or downgraded to v1.63), event
+replication will resume as normal.</p>
 <h1 id="upgrading-to-v1620"><a class="header" href="#upgrading-to-v1620">Upgrading to v1.62.0</a></h1>
 <h2 id="new-signatures-for-spam-checker-callbacks"><a class="header" href="#new-signatures-for-spam-checker-callbacks">New signatures for spam checker callbacks</a></h2>
 <p>As a followup to changes in v1.60.0, the following spam-checker callbacks have changed signature:</p>