you cannot change the customer after the shipment has been created.   delete the shipment then change the customer on the sales order then recreate the shipment.   The sales order, shipment and invoice must display the same account for tracking.

Thank you for your kind reply. Due to audit reasons, we don't want to delete the shipment. So our idea is to use the shipment for another customer. I have found an solution for this:

We need to delete all the lines in the shipment. Then customer field will open for editing, there we can change the customer code and select the desired sales order of that customer and save the shipment. This will solve our problem as we don't wan to delete shipment note.