diff --git a/src/ServiceDiscovery/SlowControlCollection.cpp b/src/ServiceDiscovery/SlowControlCollection.cpp
index f44a9aed..906599f7 100644
--- a/src/ServiceDiscovery/SlowControlCollection.cpp
+++ b/src/ServiceDiscovery/SlowControlCollection.cpp
@@ -737,28 +737,21 @@ bool SlowControlCollection::Update(SlowControlCollection* SCC, std::string key,
       //printf("d0 %s = %s : %s\n", reply.c_str(), key.c_str(), value.c_str());
       if(value!=""){
         if(!testing || (testing && !(*SCC)[key]->Lockable())){
-          
-          if(!(*SCC)[key]->SetValue(value)){
+
+          std::string change_result;
+          if(!(*SCC)[key]->SetValue(value, change_result)){
             reply =" Error setting "+key+" to value: " + value;
             return false;
           }
           else{
-            reply = value;
+            // A change function's actual return (e.g. ExportConfig's JSON
+            // payload) matters for some controls - fall back to echoing
+            // value for the common case of no change function / nothing
+            // returned, matching the old behaviour.
+            reply = change_result.empty() ? value : change_result;
             return true;
           }
           //(*SCC)[key]->Print();
-          /*
-          SCFunction tmp_func= (*SCC)[key]->GetChangeFunction();
-          if (tmp_func!=nullptr){
-            try{ 
-              reply=tmp_func(key.c_str());
-              
-            }
-            catch(...){
-              reply= "change function failed";
-            }
-          }
-          */
         }
         else reply = key + " locked";
       }
diff --git a/src/ServiceDiscovery/SlowControlElement.cpp b/src/ServiceDiscovery/SlowControlElement.cpp
index fad1e466..1db40533 100644
--- a/src/ServiceDiscovery/SlowControlElement.cpp
+++ b/src/ServiceDiscovery/SlowControlElement.cpp
@@ -216,6 +216,11 @@ bool SlowControlElement::SetValue(const char value[]){
 
 
 bool SlowControlElement::SetValue(std::string value){
+  std::string result;
+  return SetValue(value, result);
+}
+
+bool SlowControlElement::SetValue(std::string value, std::string& result){
   mtx.lock();
 
   if(m_type == SlowControlElementType(VARIABLE)){
@@ -225,10 +230,10 @@ bool SlowControlElement::SetValue(std::string value){
     mtx.unlock();
     return SetValue(val);
   }
-  
+
   if(m_change_function!=0){
     try{
-      m_change_function(value.c_str());
+      result = m_change_function(value.c_str());
     }
     catch(...){
       std::cerr<<"failed to call change fucntion"<<std::endl;
@@ -236,9 +241,9 @@ bool SlowControlElement::SetValue(std::string value){
        return false;
     }
   }
-  
-  
-  
+
+
+
   if(m_type == SlowControlElementType(INFO)){ //sanitising for web printout
     for(unsigned int i=0; i<value.length(); i++){
       if(value.at(i)==',') value.at(i)='.';
diff --git a/src/ServiceDiscovery/SlowControlElement.h b/src/ServiceDiscovery/SlowControlElement.h
index b7aa06d8..bb790a36 100644
--- a/src/ServiceDiscovery/SlowControlElement.h
+++ b/src/ServiceDiscovery/SlowControlElement.h
@@ -33,6 +33,10 @@ namespace ToolFramework{
     bool SetValue(const char value[]);
     bool SetDefault(std::string value);
     bool SetValue(std::string value);
+    // Same as SetValue(std::string), but also hands back whatever the change
+    // function actually returned (empty if there's no change function, or it
+    // returned nothing) - SetValue(std::string) alone discards that.
+    bool SetValue(std::string value, std::string& result);
     bool GetValue(std::string &value); 
     bool Lockable();
     bool Hidden();