I work for them.
while I haven't yet redeemed my Walk in the Park voucher (I have an AP instead), my general understanding is that you would basically have what amounts to a 1 day-1 park ticket to apply towards the GATE cost of another ticket type. This would be the same as if you purchased your ticket at the gate and then decided to upgrade it later in the day.
The information on the Ticket discounts we get indicate that those discounts are only available online thru the portal website, So that means you could not get those discounts at the park itself.
That being said, I DO know and can say with certainty that you can hop online while you are down there to the portal website if you have internet access, Purchase your tickets at the discounted rate, and then pick them up at the kiosks at the park entrance that same day. While I have an AP, I did bring some friends with me on my last trip who were interested in going and I was able to get them the discounted tickets that morning before we drove to the parks.
If you have any more questions I'll be happy to answer them for you. Feel free to PM me.