I am pretty sure to get the dining plan you need to have tickets for your entire stay. But honestly they do go down greatly in price the longer you are there so day. I am pretty sure that falls under the time frame of the promo I got (that 30%,35%, 40%one.)
Try quoting price for room only and see if you call vs. online - maybe you have the pin associated with your name- ask about it! Explain what you are trying to do and see if they can work a better deal for you.