Sorry that happened to you. I can't stand when a negative experience with a CM on the phone dampers our enthusiastic planning. While I have had many many great cms on the phone, I have had a couple of stinkers, too, regardless of how good a mood I am in when I call.
I try to avoid calling Disney whenever possible and do most everything online, but when I must call, I follow the advice I got here on the DIS a few years back. When I need to cancel and rebook: Call Disney to cancel. End the call and then rebook online, if possible. That way, you have as little to do as possible with CMs on the phone. It might not work for everyone, but it works for me.