Try to use this code
@api.onchange('date_order')
def change_currency_rate(self):     
    for line in self.order_line:  
    val=product_id_change(line)

def product_id_change(self,line):
----code

I know that i have to this but i don't know which of these arguments i have to pass. This is def i want to call:

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):

You have to pass the parameter to the product_id_change function 

data = line.product_id_change(self._cr, self._uid, [], pricelist, product, qty, uom, qty_uos, uos,name,partner_id,lang, update_tax, date_order, packaging, fiscal_position, flag, context=self._context)