This is what I did: I booked a room-only online at disney.com, knowing it's a very busy week (president's week) that I'm booking for, so at this point there's no discounts.
Then, I requested a quote at a couple of the TA's recommended here (chose small world). Their quotes were the same as disney's. I asked what to do if I already have an existing reservation with disney and she sent a form to fax to disney, giving disney permission to transfer my reservation to the TA. Or, she said, I could cancel that one if I was booking a package rather than room-only. I faxed the form yesterday...
Now, I'm just awaiting confirmation from the TA that all is well with my reservation - but, I am a little nervous because at this point it looks like my ressie is GONE online and I haven't yet received confirmation from theTA!