I have done the upgrade option several times for our summer vacation flights (as it was cheaper to book economy+upgrade than business directly). Flights will change to new booking class (had P, Z, D already), and upgrade fare depends on your current and your future booking class difference (I assume).
However note: the conditions of your current ticket remain - i.e. if you had an HBO economy fare, this will remain HBO (similar to rebooking and reimbursement conditions...).