HI Juan,

In Odoo 8 Api following example help you to basic understand of how's work on change method and you not need to write in xml file.

my.py

class DetalleFactura(models.Model):
    _name = "openacademy.detalle"

    @api.onchange('producto_id')
    def onchange_producto_id(self):
        # Your Code but if you want to use any field use with self.producto_id like that also you can use all fields using self.<your field name>
        # About return type in this you can not return any value but you simply set your value like
        # self.precio = 100.00
        self.precio = 100.00

    name = fields.Char(string="Titulo")
    producto_id = fields.Many2one('product.product', 'Producto')
    precio = fields.Float(string="Precio", readonly=True)

my.xml

<tree editable="bottom">
  <field name="producto_id"/>
  <field name="precio" />
</tree>

if you have to learn about new API then visit official documentation for Odoo 8 Here
 

Best,
Harsh Dhaduk