Okay, 2 thoughts on this;
1) I have a room only booked at a discount for our upcoming trip. The CM told me I have up until 3 days before to add tickets or dining plan. She did not say I would lose my discount (only 20%).
2) If for some reason you cannot add the dining, I wouldn't sweat it. I did an elaborate cost comparison after trying to decide whether we wanted the dining plan or not. First off I booked us with our equivalent of table service meals and then went through the menus and prices taking into account of who would probably get what and adding in drinks and desserts where I thought we would actually want dessert. If in doubt I went with the higher priced items. Next I figured where we may go for an equivalent amount of quick service meals and pricing there. Again, you need to add in drinks, or whatever. Third, I added in the allowance our family would receive for our snack credit a day each. And finally I figured in the price of the mugs.
The price was cheaper for us by about 250.00. I did not add in tips, as they are not added in to the what the dining plan would absorb. Also, I did not add extra snacks that were not part of the snack credit allowance.
Now the reality is that we will probably actually spend less, because of eating habits of our group, more snacking, less big meals, but for cost comparison purposes, I wanted to see. The only thing with this is some people say they have a hard time actually spending that much on your sit down meals. But if you realize that you would've paid that and more and it's budgeted in, you can't worry about it. We are keeping our sit down meals. Well, there's one I was iffy about so I may cancel that one, but other than that, don't fret! Certainly don't give up your awesome discount for a "perceived" discount.