Hi you can not use product_id_change in sale.order. Because it is in sale.order.line

Sale.order = the header Sale.order.line = a detail

So in sale.order.line you generates a field with product_id_change ans in sale.order you change your field note in function.

the principle of this function :

she travels sale.order.line your field x_discount, and affect in the header "note".

You can set note field of sale.order model by overriding product_id_change() method.

Here is the example,

def product_id_change(self, cr, uid, ids, .. ):
    vals = super(your_class, self).product_id_change(cr, uid, ids, uom_id, uom_po_id)
    note = "My Sale Order note"
    vals = {'value':{'note':note}}
    return vals

thank you sir but here is my code and it dont seems to work

class sale_order(osv.osv):

_name='sale.order'
_inherit='sale.order'

def product_id_change(self, cr, uid, ids, pricelist, product, qty=0,
    uom=False, qty_uos=0, uos=False, name='', partner_id=False,
    lang=False, update_tax=True, date_order=False, packaging=False, fiscal_position=False, flag=False, context=None):
    vals = super(sale_order, self).product_id_change(cr, uid, ids, pricelist, product, qty=qty,
    uom=uom, qty_uos=qty_uos, uos=uos, name=name, partner_id=partner_id,
    lang=lang, update_tax=update_tax, date_order=date_order, packaging=packaging, fiscal_position=fiscal_position, flag=flag, context=context)
    note = "Mys Notes"
    vals.update({'note':note})
    return vals

sale_order()