Isn't I class reward Biz?

Unless they are very, very heavily overbooked, airlines very rarely upgrade reward tickets. Of course, it has been known to happen from time to time, but normally a non-status paid passenger will probably get the upgrade before a status passenger with a reward ticket.

In any case, you might be better off asking at checkin to upgrade using cash, because to my knowledge you are able to pay USD650 for a one-class upgrade from any fare class, including rewards.

I wouldn't count on the Star upgrades, though it might be saving them just on the very off chance they are desperate for a reason to upgrade you during an overbooking situation. But good luck anyhow