DiD S11 & S12: transform all Mages instead of changing their advancements

This has the advantage that they use the same help page as newwly
recruited ons, and their advancement options are correctly shown there.

The transformation code is taken from SotA S18

[ci_skip]

data/campaigns/Descent_Into_Darkness/scenarios/11_Descent_into_Darkness.cfg

@@ -520,8 +520,65 @@
         [/remove_item]
 
         # Previously recruited and new adepts can now also become liches
-        {MODIFY_UNIT (type=Dark Adept DiD)    advances_to (Dark Sorcerer)}
-        {MODIFY_UNIT (type=Dark Sorcerer DiD) advances_to (Lich,Necromancer)}
+        # To tranform them, all units from the recall list get copied
+        # and replaced by a new one with the changed type
+        [store_unit]
+            [filter]
+                type=Dark Adept DiD, Dark Sorcerer DiD
+            [/filter]
+            variable=mages
+            kill=yes
+        [/store_unit]
+
+        [for]
+            array=mages
+            variable=i
+            [do]
+                # Get the new type:
+                [switch]
+                    variable=mages[$i].type
+                    [case]
+                        value=Dark Adept DiD
+                        [set_variable]
+                            name=new_type
+                            value=Dark Adept
+                        [/set_variable]
+                    [/case]
+                    [case]
+                        value=Dark Sorcerer DiD
+                        [set_variable]
+                            name=new_type
+                            value=Dark Sorcerer
+                        [/set_variable]
+                    [/case]
+                [/switch]
+
+                # Create a new mage with the same attributes as the old one:
+                [unit]
+                    type=$new_type
+                    max_experience=$mages[$i].max_experience
+                    max_hitpoints=$mages[$i].max_hitpoints
+                    experience=$mages[$i].experience
+                    name=$mages[$i].name
+                    id=$mages[$i].id
+                    gender=$mages[$i].gender
+                    side=1
+                    placement=recall  # The only place they can be at the moment.
+                    [modifications]
+                        [insert_tag]
+                            name=trait
+                            variable=mages[$i].modifications.trait[0]
+                        [/insert_tag]
+                        [insert_tag]
+                            name=trait
+                            variable=mages[$i].modifications.trait[1]
+                        [/insert_tag]
+                    [/modifications]
+                [/unit]
+            [/do]
+        [/for]
+
+        {CLEAR_VARIABLE mages,new_type}
 
         [disallow_recruit]
             side=1