Oosterdam Cabin Category Question

[sonoma03]

I just booked a category M inside cabin on the Oosterdam for a March cruise (specific cabin, not a guarantee). The travel agent asked me if I wanted to allow for a possible complimentary upgrade from HAL. It sure would be nice to get an upgrade to a larger cabin. However, I have noticed that some supposedly higher categories seem to be less desirable cabins. For example, some are the same size as category M but they are located at the very front or very back of the ship. Some are also right next to the elevators or stairs, which I would prefer to avoid due to possible noise. So I am puzzled about why those are considered to be higher cabin categories. What is the criteria for determining that?

I suppose the chances of a free upgrade are slim anyway, but does anyone know if you have the choice of refusing an upgrade and staying in your original cabin if you don't like the new one?

Thanks!