Assuming the change fee is $150 and the cancellation fee is $200, if you want to upgrade to business, it'd be best to pay $150 for the change in addition to the extra miles required. Unfortunately, there's no way to just pay the additional miles because upgrading a segment technically requires an entirely new ticket. Also, all the other segments must have availability (as if you're making a new booking). Now if you're lucky and get a free upgrade at the airport on the day of, that should be fine since the operating carrier has control of the ticket starting from 24 hours before departure.