There is an old racing adage that as a racehorse owner you should keep yourself in the best company and your horse in the worst. The statistics are pretty clear on this. Horses have the best chance of winning if they either compete in the same class of race than they did before or take a drop in grade. 
 
From a punting point of view it would be sensible to stick to horses that are either competing in the same grade of race as they did before or competing in a lower class of race. However, this would be a pretty obvious strategy. Could you really make a profit by focussing on only horses that either kept to the same race class or took a drop in class? The answer would appear to be 'yes'.