Is your current reservation showing as available with the Disney visa offer? The resort, room type & length of stay would have to be available to get the discount. Just having a room already booked during the offer dates & paying with the visa won’t get you the discount. The offer has limited availability, just as all Disney offers do.
If it is, sure, I’d call & see if they can apply the discount. If they say no, you can probably cancel that reservation & rebook. Just be aware that you would need a new deposit that way. And possibly increase your ticket costs if those were included in your current reservation.