It boils down to this:

If you want an upgrade, but there is none available at the time you book a ticket, you can wait list for one. If there is one available, then just book it. There will be a fee for this service (which depends on the fare bucket you book into) and miles will be deducted from your account at the time you ask for the upgrade. If the upgrade does not clear and you travel in the back of the bus, then the fee and the miles will be returned to you. I don't think its much more complicated than that. Generally award availability is first posted at 330 days in advance and is periodically updated. I assume upgrade availability follows the same schedule. Good luck as the trip will be much more comfortable up front and if your dates are flexible, you should have no problems finding availability. ^