diff --git a/blocks.js b/blocks.js
index af5542f7..159d1a20 100644
--- a/blocks.js
+++ b/blocks.js
@@ -2009,6 +2009,7 @@ BlockMorph.prototype.userMenu = function () {
     var menu = new MenuMorph(this),
         world = this.world(),
         myself = this,
+        alternatives,
         blck;
 
     menu.addItem(
@@ -2066,6 +2067,14 @@ BlockMorph.prototype.userMenu = function () {
                 );
             }
         );
+    } else if (this.definition && this.alternatives) { // custom block
+        alternatives = this.alternatives();
+        if (alternatives.length > 0) {
+            menu.addItem(
+                'relabel...',
+                function () {myself.relabel(alternatives); }
+            );
+        }
     }
 
     menu.addItem(
diff --git a/byob.js b/byob.js
index 0f79acfd..a17f9768 100644
--- a/byob.js
+++ b/byob.js
@@ -106,7 +106,7 @@ SymbolMorph, isNil*/
 
 // Global stuff ////////////////////////////////////////////////////////
 
-modules.byob = '2014-Jun-04';
+modules.byob = '2014-Jun-06';
 
 // Declarations
 
@@ -745,6 +745,7 @@ CustomCommandBlockMorph.prototype.userMenu = function () {
         } else {
             menu.addLine();
         }
+
         // menu.addItem("export definition...", 'exportBlockDefinition');
         menu.addItem("delete block definition...", 'deleteBlockDefinition');
     }
@@ -846,6 +847,44 @@ CustomCommandBlockMorph.prototype.popUpbubbleHelp = function (
     ).popUp(this.world(), this.rightCenter().add(new Point(-8, 0)));
 };
 
+// CustomCommandBlockMorph relabelling
+
+CustomCommandBlockMorph.prototype.relabel = function (alternatives) {
+    var menu = new MenuMorph(this),
+        oldInputs = this.inputs().map(
+            function (each) {return each.fullCopy(); }
+        ),
+        myself = this;
+    alternatives.forEach(function (def) {
+        var block = def.blockInstance();
+        block.restoreInputs(oldInputs);
+        block.fixBlockColor(null, true);
+        block.addShadow(new Point(3, 3));
+        menu.addItem(
+            block,
+            function () {
+                myself.definition = def;
+                myself.refresh();
+            }
+        );
+    });
+    menu.popup(this.world(), this.bottomLeft().subtract(new Point(
+        8,
+        this instanceof CommandBlockMorph ? this.corner : 0
+    )));
+};
+
+CustomCommandBlockMorph.prototype.alternatives = function () {
+    var rcvr = this.receiver(),
+        stage = rcvr.parentThatIsA(StageMorph),
+        allDefs = rcvr.customBlocks.concat(stage.globalBlocks),
+        myself = this;
+    return allDefs.filter(function (each) {
+        return each !== myself.definition &&
+            each.type === myself.definition.type;
+    });
+};
+
 // CustomReporterBlockMorph ////////////////////////////////////////////
 
 // CustomReporterBlockMorph inherits from ReporterBlockMorph:
@@ -961,6 +1000,14 @@ CustomReporterBlockMorph.prototype.bubbleHelp
 CustomReporterBlockMorph.prototype.popUpbubbleHelp
     = CustomCommandBlockMorph.prototype.popUpbubbleHelp;
 
+// CustomReporterBlockMorph relabelling
+
+CustomReporterBlockMorph.prototype.relabel
+    = CustomCommandBlockMorph.prototype.relabel;
+
+CustomReporterBlockMorph.prototype.alternatives
+    = CustomCommandBlockMorph.prototype.alternatives;
+
 // JaggedBlockMorph ////////////////////////////////////////////////////
 
 /*
diff --git a/history.txt b/history.txt
index 97b487e0..02fc2a00 100755
--- a/history.txt
+++ b/history.txt
@@ -2165,3 +2165,4 @@ ______
 * Blocks, objects: enable relabelling blocks with C-Slots
 * Blocks: enable relabelling blocks across categories
 * Objects: more relabelling options for SAY, THINK, ASK
+* BYOB, Blocks: relabelling custom blocks (experimental)