So I created a Server Script as suggested by @Pawan.
Script Type : DocType Event
Reference Document Type : Sales Order
DocType Event : Before Save (Submitted Document)
and wrote the following code as per examples give in the previous post.
from erpnext.selling.doctype.sales_order.sales_order import make_sales_invoice
current_date = doc.transaction_date;
so = doc;
if so.docstatus==1 and not so.per_billed:
si = make_sales_invoice(so)
si.doc.posting_date = current_date
Unfortunately, I get the following error when a Sales Order is updated.
ImportError: __import__ not found
It seems that I am very close to the solution. I would be obliged if anyone (especially @Pawan) can help to fix that probelm.