Yes you do, last I checked it is the same savings as you would get as you see on the website (however this was before the recent price increase... but I am assuming they are still the same).
But yes, even though the
Disney Store is not owned by Disney any more, you
CAN still upgrade tickets while at the parks with-in the first 14 days of the first use (and in fact, up until the last month or so, you could only add on the no exp option for most of the tickets at the park). I am sorry klwdisney that it happened to you, but that Cast Member you had talked to was mistakened.
But I do want to stress that if you do buy your tickets at TDS, make sure you check your receipt and make sure it has the correct amout of days/the options you want on the ticket BEFORE you leave the store. After you leave the store and your ticket is not the correct one you had asked for it is pretty much impossible to correct it until you get to the parks. This does not happen often, but even the best of TDS CMs will make a mistake once in awhile (even more so when all the pages the CMs scan look pretty much the same)