Fixing: U4-6942 During upgrade MUST make sure that the new Property Type GUIDs are consistent ... just needs some testing now;

src/Umbraco.Core/Persistence/Migrations/Upgrades/TargetVersionSevenThreeZero/AddUniqueIdPropertyTypeColumn.cs

@@ -30,9 +30,26 @@ namespace Umbraco.Core.Persistence.Migrations.Upgrades.TargetVersionSevenThreeZe
                     .NonClustered()
                     .WithOptions()
                     .Unique();
+
+                //now we need to fill in the data so that it is consistent, we can't have it generating random GUIDs for 
+                // the already existing data, see: http://issues.umbraco.org/issue/U4-6942
+
+                foreach (var data in Context.Database.Query<dynamic>(@"
+SELECT cmsPropertyType.id ptId, cmsPropertyType.Alias ptAlias, cmsContentType.alias ctAlias
+FROM cmsPropertyType
+INNER JOIN cmsContentType
+ON cmsPropertyType.contentTypeId = cmsContentType.nodeId"))
+                {
+                    //create a guid from the concatenation of the property type alias + the doc type alias
+                    string concatAlias = data.ptAlias + data.ctAlias;
+                    var ptGuid = concatAlias.ToGuid();
+
+                    //set the Unique Id to the one we've generated
+                    Update.Table("cmsPropertyType").Set(new {uniqueID = ptGuid}).Where(new {id = data.ptId });
+                }
             }
 
-            
+
         }
 
         public override void Down()

src/Umbraco.Core/StringExtensions.cs

@@ -1409,5 +1409,18 @@ namespace Umbraco.Core
         {
             return string.IsNullOrEmpty(text) ? text : InvalidXmlChars.Replace(text, "");
         }
+
+        /// <summary>
+        /// Converts a string to a Guid - WARNING, depending on the string, this may not be unique
+        /// </summary>
+        /// <param name="text"></param>
+        /// <returns></returns>
+        internal static Guid ToGuid(this string text)
+        {
+            var md5 = MD5.Create();
+            byte[] myStringBytes = Encoding.ASCII.GetBytes(text);
+            byte[] hash = md5.ComputeHash(myStringBytes);
+            return new Guid(hash);
+        }
     }
 }