Try using something besides a slash for your replacement delimiter so it's easier to read and that so you won't have to escape the slash you are searching for.
sed does not care what it is as long as you use three of a kind.
Code:
sed -e 's|/9|/8|g;'
See:
However, you might want to think out the logic of how you are going to replace all 10 digits one at a time. You'll probably need a non-digit place holder for one of them.