Congratulations on achieving Top-Rated.
You won't see the amount of the FVF (Final Value Fee) discount applied to your monthly sales until your billing period ends and ebay calculates it, the discount.
My billing period ends on the last day of the month so around the 2nd or 3rd of the next month, my invoice will be ready for viewing and it will have the Top-rated discount applied. It will say exactly how much it has been reduced. Note the FVF on Shipping is not included in discountable final value fees.
Then, if you are like me, your bill will due due around the middle of that next month so if there are any problems to attend to, you have ample time to call Customer Service.
I'm thinking it will apply to all the listings sold from the period of time you became Top-rated and not before but I could be incorrect about that.
Another seller here may know better.