I want to add a new field in a model. I create it in the python file:

class SaleOrderExt(models.Model):
    _inherit = ['sale.order']
    my_new_field = fields.Char()

and then I am going to show it in the view form:

<?xml version="1.0" encoding="utf-8"?>
<openerp>
<data>
  <record id="view_order_form_extend" model="ir.ui.view">
  <field name="name">view.order.form.extend</field>
  <field name="model">sale.order</field>
  <field name="inherit_id" ref="sale_stock.view_order_form_inherit_sale_stock"/>
  <field name="arch" type="xml">
  <xpath expr="//field[@name='picking_policy']" position="after">
    <field name="my_new_field"/>
  </xpath>
  </field>
</record>
</data>
</openerp>

When I install the module for the first time, the field appears and everything works fine. The problem comes when I try to update the module. 

It gives an exception that says that field product_tmpl_id doesn't exist.

How can it be possible? The first installation worked fine...