Wow. For a thread asking for the "best case scenario", ya'll are really bumming me out here.
Best case: MS makes this painless. As I have posted before on these threads, this all comes down to 1 basic philosophy:
Either MS keeps the creative teams together that made Nokia the device maker it is, or they lose these people. It isn't about the name, it is about the engineers, the designers, and the production teams. Keep those people in place, and all will be good in Microsoft Mobility. Lose them, and while it may not be doomsday, it just won't be the best case scenario.
Losing the brand recognition of Nokia will hurt, but "Lumia" has become a recognized name as well. Stick to that, and buyers will still be aware of your product.
I am cautiously optimistic.