The dining plan itself is pretty much never discounted. You can get discounted package deals that include the dining plan, such as "free" dining and Stay, Play, and Dine.
You can always change to a discounted package when they are released if there is one for your dates. However, keep in mind that if your current stay is a room-only reservation, you'll have to pay an additional deposit of $200 for the package reservation, then wait on Disney to refund the original amount you've paid thus far.