This is a great option to have! I emailed Fosi Support (Carl) about my concern about upgrading my order to a different product. Without any blowback, Carl stated to submit "X" amount using the 'Make Up the Price Difference of $1' option on Fosi's website. This basically allowed me to upgrade my order from the V3 to ZA3 without personally going through the hassle of canceling my order and reordering. I just paid the difference and recieved shipping information the following day. It was pretty easy.
Thanks Fosi Audio Team!