Is it easier to update the time of the Fastpass+ that I already have than trying to get one in the first place or do they just come available for everyone regardless whether you already have one for that attraction?
I currently have 4 fastpass+ for Avatar Flight of Passage on 9/5/17 (4:10 pm - 5:10 pm). I would like to trade my 4 9/5/17 fastpass+ for 4 9/2/17 fastpass+ for Avatar Flight of Passage. I made my resort reservations well in advance and booked my fastpass+ early morning on the 60th day out; however, I have since had to shorten/move my vacation dates due to my daughter's college class schedule. I was able to re-work all of my fastpasses except Avatar Flight of Passage. I am still holding the 9/5/17 fastpass+ for Avatar Flight of Passage hoping that I can trade it or a cast member at Disney will let me switch it. I asked the cast member on the phone when I changed my resort reservations and they weren't able to help me. ;(
In the case of 7DMT, your daughter has a better chance of getting a single FP than you will have modifying your existing time. Single FPs for extremely popular attractions like 7DMT are easier to get day before or day of (as people cancel.) If you try to modify your time you will probably get the "there are no more times available" message.
This applies to Frozen Ever After and Flight of Passage as well. Any other attractions in the parks are much easier to modify.
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