Errr… to check instant upgrade availability is easy… you just search on line and see if business reward seats are available.
If they are, it’s an instant upgrade via phone call.
It’s possible the seats identified on your first call were taken in the meantime.
The best way to do it is to book in real time - have the booking page open, see the business class award seat available, book and pay for economy, and instantly call the call centre to upgrade.
The call centre agent is incorrect that you can only request an upgrade seven days before departure. You can request a complimentary confirmed upgrade at any time you see a business class award seat become available.