Modele 8

I am moving back to the U.S. later this week and someone told me that I need to go to my commune (Brussels) to de-register (turn in my ID card) and receive a Modele 8 document. My question is this: what are the consequences if I do not do this? I have a very busy week with packing, moving, etc, and don't particularly want to spend half a day in line at the commune if not absolutely necessary. Any advise would be welcomed.

Thanks in advance,