Changing the bodies pH would not be a good thing. Our bodies biological processes are pH dependent. There is a very tight range of pH that our bodies HAVE to maintain for us to live. Because of that tight range in pH our bodies have various ways of compensating for any potential change brought about by food, drink, or otherwise.
That is not to say that intentionally putting something into the body that would affect pH is not a good or bad idea. For instance, I've seen some evidence that suggests a diet high in protein may correlate positively with a reduction in bone density. The reasoning behind this is that to compensate for the decrease in pH caused by consumption of excess protein the body leaches calcium carbonate from the bones.
Another example is ketoacidosis, a life threatening condition that can be brought about by untreated diabetes. Being unable to metabolize sugars to make energy the body needs, the body starts breaking down fatty acids. A treatment involving infusion of calcium carbonate directly into the blood to raise pH levels has been widely used, but is controversial due to conflicting evidence of efficacy.
As far altering the pH of the body to help 'treat' any disease or potential disease... I have seen no evidence suggesting it is beneficial. I have some some evidence suggesting it may be potentially harmful though. Any entity trying to sell such a product or making any such claims is complete quackery in my opinion.