Because you are paying for the dining plan, you will be able to keep your room discount. However, you will have to purchase at least a one day ticket for everyone in the room. It now becomes a package (room + ticket + dining plan) so all of the deposit/final payment/cancellation terms for a package will apply. Also, everyone in the room will have to be on the same dining plan.
One possible snag is that changing from a room only reservation to a package might mean you will have to make a completely new reservation. If that's the case, make sure there is a room available with your discount first.
You can have a ticketless package if you have an AP or are a DVC member staying on points.