diff --git a/src/clientmon.cpp b/src/clientmon.cpp
index d4f1aaf..03633e9 100644
--- a/src/clientmon.cpp
+++ b/src/clientmon.cpp
@@ -177,6 +177,14 @@ struct SubscriptionImpl : public OperationBase, public Subscription
         return std::shared_ptr<Subscription>(self);
     }
 
+    virtual void onEvent(std::function<void(Subscription&)>&& fn) override final {
+        decltype (event) junk;
+        loop.call([this, &junk, &fn]() {
+            junk = std::move(event);
+            this->event = std::move(fn);
+        });
+    }
+
     virtual bool cancel() override final {
         decltype (event) junk;
         bool ret;
diff --git a/src/pvxs/client.h b/src/pvxs/client.h
index 4c67f1b..b606a62 100644
--- a/src/pvxs/client.h
+++ b/src/pvxs/client.h
@@ -199,6 +199,10 @@ public:
      */
     virtual Value pop() =0;
 
+    // Expert API
+    // replace handler stored with MonitorBuilder::event()
+    virtual void onEvent(std::function<void(Subscription&)>&&) =0;
+
     //! Return strong internal reference which will not prevent
     //! implicit cancellation when the last reference returned
     //! by exec() is released.