Do not allow to change template of existing product

Many models rely on the couple product-template to be immutable.

Closes #14279

modules/product/message.xml

@@ -33,5 +33,8 @@ this repository contains the full copyright notices and license terms. -->
         <record model="ir.message" id="msg_category_code_unique">
             <field name="text">The code on product category must be unique.</field>
         </record>
+        <record model="ir.message" id="msg_product_change_template">
+            <field name="text">You cannot change the template of the product "%(product)s".</field>
+        </record>
     </data>
 </tryton>

modules/product/product.py

@@ -16,6 +16,7 @@ from trytond.i18n import gettext
 from trytond.model import (
     DeactivableMixin, Exclude, Index, Model, ModelSQL, ModelView, UnionMixin,
     fields, sequence_ordered, tree)
+from trytond.model.exceptions import AccessError
 from trytond.modules.company.model import (
     CompanyMultiValueMixin, CompanyValueMixin)
 from trytond.pool import Pool
@@ -717,6 +718,17 @@ class Product(
         if mode in {'create', 'write'}:
             cls.sync_code(products)
 
+    @classmethod
+    def check_modification(cls, mode, products, values=None, external=False):
+        super().check_modification(
+            mode, products, values=values, external=external)
+        if mode == 'write' and 'template' in values:
+            for product in products:
+                if product.template.id != values.get('template'):
+                    raise AccessError(gettext(
+                            'product.msg_product_change_template',
+                            product=product.rec_name))
+
     @classmethod
     def copy(cls, products, default=None):
         if default is None: