Thanks Robo, supersnoop and everyone. I guess I should have went with an authorized reseller (for the discount) even though at the time we only needed 2 days.

So now we want to add a third day to the beginning of our trip. That won't goof up the FPs we already have will it? So for clarity, we arrive on the 13th, and were planning on the parks only on the 16th and 17th (FPs already reserved). Now we want to add a park day to the 15th. I know I can't get FPs for that day until we arrive and add the day. This is my first time adding days, it's an unusual trip for us!