You should not have to worry about losing your room. The CM would not cancel the first reservation until you were sure that you were booking a new one. Of course if you are almost through the process of booking the new one and decide to change your mind and want the original one back, there is a slim possibility that it could be gone. But most likely not.
One more thing. You actually can book 499 days in advance, not 500, but that is 499 days from your last night at disney. The days get released one day at a time. So while your arrival date may be 499 days away and a room can be booked for it, you need to wait until your departure date has been released so you can book the whole vacation at once.