You are going to read replies that say that you will retain the discount and those that say you will not. You will likely read that you have to use the ticket first in order to retain the discount and others that say you do not. Some people will say that if you do get to retain the discount, it is "pixie dust" and should not be expected.
We went down in September and brought with us one 3 day Parkhopper (the actual ticket, not a voucher) that I got free by using Airmiles and two 4 day parkhopper vouchers that I bought at a discount from LMTClub. We went right to the ticket booth since we knew we were going to upgrade. I was able to upgrade all three of the hoppers to Deluxe APs and I was credited the full gate price for each of them, so I did retain the discount. I was credited $295 for the 3 day hopper and $320 for each of the 4 day hoppers and only paid the difference even though I only paid $275.25 for each of the 4 day hoppers and the 3 day was $0 cash (used Airmiles to get it).
I've upgraded from discount passes to APs once before and was able to retain the discount, I've also added a day onto a discount hopper and was also able to retain the discount. Maybe I've been super lucky and have been granted a big helping of pixie dust and YMMV.