I'm not sure you're following my logic. I also realize I didn't make my thought clear enough. Also, I didn't see before this post where you said how long you had the Surface. Did you buy it one month before the AU was released, or one month from now? Because the AU has been out for almost a month.
Anyway, I'll try to describe my thought pattern more clearly.
When you first bought your device, it worked fine. In the time between when you bought it and the AU was released, your Surface went from being new to being 1 month old (again, I'm not sure on the timing here).
Enter the Anniversary Update. Suddenly your 1-month-old-working-perfectly-with-all-current-updates Surface turned into a device needing a warranty replacement, due to issues with the Anniversary Update. Since you had no issues with it until the AU, warranty should only be obligated to give you a similar device to what yours was at the point it started having problems, except of course this one must actually work with the AU. Warranty's obligation is to provide for you a 1-month-old-working-perfectly-with-all-current-updates device.
Now, if you would have had problems with it from Day 1, I can see why you should get a new device. But if you would have had problems right away, you would have had it replaced earlier.
Either way, I hope you get your new device. Because it is only one month old you probably have a better chance. It sounds like you will have to request it though!