it would change the amount of rest that you would be required to receive but they could reduce it and give you comp rest for the next day.
If I'm understanding the rules correctly, it wouldn't even do that

! Yeah...it could effect the amount of rest, but it's not going to "bump you up". The only change overblocking will result in is a reduction of your rest, because they're going to take you to min rest if you're not already there (assuming it will help maintain the schedule). If you were originally scheduled for 8 hours of min rest, you still only get 8 hours min rest even if you were scheduled for 7.5 hours but actually blocked 9 hours with the same schedule. Whether you get 9 hours or 8 hours for min rest is all based on your scheduled block, looking back over the 24 hours preceding the scheduled beginning of the rest period.
Actual block can only affect the time that said rest periods begins (i.e. you might have to delay the flight the next morning), but won't affect its duration (i.e. you don't suddenly get 9 hrs instead of 8 because you over-blocked). Rest period duration is pre-determined by scheduled block. Where it can get "all jacked up" is when you're on reserve or are reassigned and start getting extra legs thrown on your schedule. Now, your actual block for the day starts to affect your scheduled block. Sometimes, I wish I would've paid attention in Calculus II.
Go into FAR 121.471 and notice how many times it says "Scheduled" and how many times it says "Actual". (Hint: the score is 14 to 0)
With that said, we'll soon be able to forget all of the above, as it's going to have a wrecking ball taken to it very soon by the feds.
Disclaimer: It's certainly possible that I'm wrong, so anyone feel free to correct me. It seems like I learn something new about rest rules every year, if not every month.