My advice on this is just to keep trying until you get someone who can do what you want. I'm not sure about the keeping the discount part, but I called yesterday to make a (rack rate) room-only reservation into a package. The first CM told me there was "no availability" for my resort, even though I already had a room booked.
He explained that room-only reservations and package reservations are "pooled" differently, so he couldn't book a package since there were no "package rooms" to book.
Anyway, I hung up with him and called back. The 2nd CM told pretty much the same thing, but put me on hold while he talked with Guest Services and then came back to tell me the same thing. When I asked to speak to GS, finally they were able to book a package and then cancel my room-only. It took me 40 minutes talking to 4 people to do, but it worked.
My concern would be trying to keep a discount. If that offer has expired or is not available, you might run into some issues. So just keep trying and hopefully you'll eventually get someone who can book the package for you and keep your discount.