Hey Brian, this actually would have been a lot easier if those stories were still associated with Sprint 99 because then we'd know how to locate them all! I'm guessing they are probably still scheduled in some way - maybe a Release?
I would probably create a Custom List and see if you can locate them all. Then, use bulk edit to remove any timebox from these stories. As long as they're not schedule to any iteration (and have no children stories), they will show up in the Backlog again.
Do the same as this -- but maybe with Release: