If the uuid of a SceneObjectGroup (RootPart) is changed before adding to the scene, remove the old uuid reference from m_parts as well as adding the new one.

The separate remove and set operations is SOG.set_UUID() are both locked under m_parts.SyncRoot since they are logically atomic (though this isn't such an issue if the SOG isn't part of a scene)
Added unit test for this behaviour.
Also changed the second m_parts.AddOrReplace() to m_parts.Add().  As the old reference is now removed we never end up replacing an identical uuid.  And if we replace a uuid that's already there (from a child part) then this is an error.

OpenSim/Region/Framework/Scenes/SceneObjectGroup.cs

@@ -330,8 +330,12 @@ namespace OpenSim.Region.Framework.Scenes
             get { return m_rootPart.UUID; }
             set 
             {
+                lock (m_parts.SyncRoot)
+                {
+                    m_parts.Remove(m_rootPart.UUID);
                     m_rootPart.UUID = value;
-                m_parts.AddOrReplace(value, m_rootPart);
+                    m_parts.Add(value, m_rootPart);
+                }
             }
         }
 

OpenSim/Region/Framework/Scenes/Tests/SceneObjectBasicTests.cs

@@ -189,5 +189,40 @@ namespace OpenSim.Region.Framework.Scenes.Tests
         //    SceneObjectPart retrievedPart = scene.GetSceneObjectPart(part.LocalId);
         //    Assert.That(retrievedPart, Is.Null);
         //}
+        
+        /// <summary>
+        /// Changing a scene object uuid changes the root part uuid.  This is a valid operation if the object is not
+        /// in a scene and is useful if one wants to supply a UUID directly rather than use the one generated by 
+        /// OpenSim.
+        /// </summary>
+        [Test]
+        public void TestChangeSceneObjectUuid()
+        {
+            string rootPartName = "rootpart";
+            UUID rootPartUuid = new UUID("00000000-0000-0000-0000-000000000001");
+            string childPartName = "childPart";
+            UUID childPartUuid = new UUID("00000000-0000-0000-0001-000000000000");
+            
+            SceneObjectPart rootPart
+                = new SceneObjectPart(UUID.Zero, PrimitiveBaseShape.Default, Vector3.Zero, Quaternion.Identity, Vector3.Zero) 
+                    { Name = rootPartName, UUID = rootPartUuid };
+            SceneObjectPart linkPart
+                = new SceneObjectPart(UUID.Zero, PrimitiveBaseShape.Default, Vector3.Zero, Quaternion.Identity, Vector3.Zero) 
+                    { Name = childPartName, UUID = childPartUuid };
+
+            SceneObjectGroup sog = new SceneObjectGroup(rootPart);            
+            sog.AddPart(linkPart);
+            
+            Assert.That(sog.UUID, Is.EqualTo(rootPartUuid));
+            Assert.That(sog.RootPart.UUID, Is.EqualTo(rootPartUuid));
+            Assert.That(sog.Parts.Length, Is.EqualTo(2));
+            
+            UUID newRootPartUuid = new UUID("00000000-0000-0000-0000-000000000002");
+            sog.UUID = newRootPartUuid;
+                        
+            Assert.That(sog.UUID, Is.EqualTo(newRootPartUuid));
+            Assert.That(sog.RootPart.UUID, Is.EqualTo(newRootPartUuid));            
+            Assert.That(sog.Parts.Length, Is.EqualTo(2));
+        }
     }
 }