[FIX] stock: do not raise if writing the same value

Rewrite the product associated to a serial number.
There is a check to forbid this if some move lines already exist.
This check does not actually care what value is written,
so it can raise uselessly (and forbid a legitimate write).

opw 2049717

closes odoo/odoo#35736

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/stock/models/stock_production_lot.py

@@ -40,7 +40,7 @@ class ProductionLot(models.Model):
     @api.multi
     def write(self, vals):
         if 'product_id' in vals:
-            move_lines = self.env['stock.move.line'].search([('lot_id', 'in', self.ids)])
+            move_lines = self.env['stock.move.line'].search([('lot_id', 'in', self.ids), ('product_id', '!=', vals['product_id'])])
             if move_lines:
                 raise UserError(_(
                     'You are not allowed to change the product linked to a serial or lot number ' +