The damage will most likely be done early in the fermentation process as that is when the yeast does most of it's work. That being said, I would also like to tell you that I have a blond that I use Nottingham on and TRY to hit a 75F temp. In some beers that little off flavor may make it. Don't worry, Relax, Have a Home brew. No matter the outcome it will still be drinkable. If you drop the temps now there will be no ill effects of doing so and will help to stop further off flavors from coming through.