You will redeem your CityPass voucher at the DL ticket booth. At that time you can upgrade your pass, but the amount of credit you will receive toward the 4-day hopper will be the advance purchase cost of the 3-day hopper (currently $149), and you will probably be charged the gate price for the 4-day hopper (currently $199), so it would cost you $50 to upgrade (as opposed to the $20 advance purchase price difference).
I understand. I may have to buy a different kind of pass upfront then. I think there is another San Diego pass I can take advantage of besides the citypass, although that Citypass is a STEAL!!!