You have a 25 on top and bottom, so they cancel. The same goes for the 3 on top and the 3 on bottom and the on top and on bottom.
When you cancel all of those you are left with
You cant change the 24s for 8s, you can only change 1 24 for 8 because there are 3 24's and only 1 3. If there were 3 3s on top then you could change all the 24s to 8s.