[IMP] mail: keep old subtype subscription on change on parent record

With this commit, when changing subtype subscription from a parent
record (e.g. project of task), then it does not update subtype
subscription of existing records. Changes on parent record should
only affect new record afterwards.

Task-Id 1974055

Closes #34150

Signed-off-by: Alexandre Kühn (aku) <aku@odoo.com>

addons/mail/models/mail_thread.py

@@ -275,7 +275,7 @@ class MailThread(models.AbstractModel):
             for key, val in self._context.items():
                 if key.startswith('default_') and key[8:] not in create_values:
                     create_values[key[8:]] = val
-            thread._message_auto_subscribe(create_values)
+            thread._message_auto_subscribe(create_values, followers_existing_policy='update')
             create_values_list[thread.id] = create_values
 
         # automatic logging unless asked not to (mainly for various testing purpose)
@@ -2824,7 +2824,7 @@ class MailThread(models.AbstractModel):
                 model_description=model_description,
             )
 
-    def _message_auto_subscribe(self, updated_values):
+    def _message_auto_subscribe(self, updated_values, followers_existing_policy='skip'):
         """ Handle auto subscription. Auto subscription is done based on two
         main mechanisms
 
@@ -2881,7 +2881,7 @@ class MailThread(models.AbstractModel):
             self._name, self.ids,
             list(new_partners), new_partners,
             list(new_channels), new_channels,
-            check_existing=True, existing_policy='skip')
+            check_existing=True, existing_policy=followers_existing_policy)
 
         # notify people from auto subscription, for example like assignation
         for (template, lang), pids in notify_data.items():