[FIX] calendar: Several fixes

* Don't overwrite renamed field `alarm_type` in noupdate changes.
* Make sure `event_tz` field is empty for preserving previous behavior.

addons/calendar/migrations/13.0.1.0/noupdate_changes.xml

@@ -2,38 +2,38 @@
 <odoo>
   <record id="alarm_mail_1" model="calendar.alarm">
     <field name="name">Email - 3 Hours</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">email</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">email</field>-->
   </record>
   <record id="alarm_mail_2" model="calendar.alarm">
     <field name="name">Email - 6 Hours</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">email</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">email</field>-->
   </record>
   <record id="alarm_notif_1" model="calendar.alarm">
     <field name="name">Notification - 15 Minutes</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">notification</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">notification</field>-->
   </record>
   <record id="alarm_notif_2" model="calendar.alarm">
     <field name="name">Notification - 30 Minutes</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">notification</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">notification</field>-->
   </record>
   <record id="alarm_notif_3" model="calendar.alarm">
     <field name="name">Notification - 1 Hours</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">notification</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">notification</field>-->
   </record>
   <record id="alarm_notif_4" model="calendar.alarm">
     <field name="name">Notification - 2 Hours</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">notification</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">notification</field>-->
   </record>
   <record id="alarm_notif_5" model="calendar.alarm">
     <field name="name">Notification - 1 Days</field>
-<!--    <field name="type"/>-->
-    <field name="alarm_type">notification</field>
+<!--    <field name="type"/>
+    <field name="alarm_type">notification</field>-->
   </record>
   <record id="calendar_template_meeting_changedate" model="mail.template">
     <field name="email_from">${(object.event_id.user_id.email_formatted or user.email_formatted or '') | safe}</field>

addons/calendar/migrations/13.0.1.0/post-migration.py

@@ -1,10 +1,20 @@
 # Copyright 2020 ForgeFlow <http://www.forgeflow.com>
+# Copyright 2020 Tecnativa - Pedro M. Baeza
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html).
 
 from openupgradelib import openupgrade
 
 
+def _empty_calendar_event_tz(env):
+    """Make sure event tz is empty for equal behavior as in v12."""
+    openupgrade.logged_query(
+        env.cr,
+        "UPDATE calendar_event set event_tz = NULL WHERE event_tz IS NOT NULL",
+    )
+
+
 @openupgrade.migrate()
 def migrate(env, version):
+    _empty_calendar_event_tz(env)
     openupgrade.load_data(
         env.cr, 'calendar', 'migrations/13.0.1.0/noupdate_changes.xml')