[FIX] sale_management: lines reset on template change

The removal of existing order lines on template update was unexpectedly
lost in recent code changes.

This commit brings back the old expected behavior.

closes odoo/odoo#82868

Signed-off-by: Victor Feyens (vfe) <vfe@odoo.com>

addons/sale_management/models/sale_order.py

@@ -87,13 +87,16 @@ class SaleOrder(models.Model):
     def _onchange_sale_order_template_id(self):
         sale_order_template = self.sale_order_template_id.with_context(lang=self.partner_id.lang)
 
-        self.order_line = [
+        order_lines_data = [fields.Command.clear()]
+        order_lines_data += [
             fields.Command.create(
                 self._compute_line_data_for_template_change(line)
             )
             for line in sale_order_template.sale_order_template_line_ids
         ]
 
+        self.order_line = order_lines_data
+
         option_lines_data = [fields.Command.clear()]
         option_lines_data += [
             fields.Command.create(