As @Didier said, you can change your delimiter to something other than /:
grep -rl $oldstring /path/to/folder | xargs sed -i s@$oldstring@$newstring@g
As @Didier said, you can change your delimiter to something other than /:
grep -rl $oldstring /path/to/folder | xargs sed -i s@$oldstring@$newstring@g