That didn't happen to me. Maybe they misunderstood your question? I had a reservation booked for the exact same days that I re-booked for online.
I kept the old reservation at first (just in case), and booked the new reservation with
free dining. I think I called later that night to cancel the old reservation once the phone lines weren't as crazy.
I used the same account number. I actually had two rooms booked that way. I was so stressed out that I was going to mess things up but everything worked out!
6am online and 7am phones is what I have heard as well. I was on hold two years ago for a few hours. I called work on another phone to let them know I was going to be late. After a couple of hours I gave up! Lol.
I think you should be fine doing what I did, but I don't want to give you the wrong advice and something doesn't work out for you. Maybe call and speak to someone else?