Worms Everywhere!!!!!
I made the mistake of asking a similar question before my Sept. trip. It got ugly on here.
When we were there in Sept., the bar codes had not been started. There are signs that state that mugs are good for the Length of Stay. That said, the CM's don't pay attention and don't say anything if you use old ones. I even saw a woman filling up 20 oz. Coke bottles!!! I don't know how someone can have the nerve to do something like that.
Do what you feel is right. Use your old ones or buy new ones ($9.99 each in Sept.). BTW, we bought new ones on our trip.