Once you add dining, it's a package now, its no longer a room only reservation (even though it is a ticketless package). The thing is that the reservation becomes subject to package rules for payment, cancellation and changes. Packages require a set deposit, and you must pay in full by a certain date, so if you are adding the dining plan after that date you would have to pay in full for it (and for any remaining balance on the room). If you're not past the pay in full date for packages, and your deposit for the room only reservation was less than the package deposit, you'll need to round up your deposit to meet package policies.