We did change our FP+ during the day or the night before, so it was possible during the busy times. However, not all attractions or time slots were open, so we just took the ones we could get, just in case.
I looked at my DD's
DAS card. It has three return times written in for the time we were there. The first one at AK, we never used due to the timing not being so good for DD, so when the second one was entered at MK the next day, the CM crossed off the AK one (as they are supposed to do). The MK one (Pooh) we used successfully two days later (there are no dates on the DAS card that we could see). The third one at AK, we did not use.
I think that the DAS may work very well for some families. It's like an extra FP for one attraction, so added to the potentially three FP+ selections, a person could see four attractions at one park on one day. This would require that the person and family could schedule fairly well and stick to their schedules (some families do a great job of this, with ADRs, etc.), or tour during off hours (plenty of attractions with no wait times after 10PM), but some families (like ours) need to be spontaneous and seize a good moment when we can, not when the FP+ time arrives. I'm not faulting the FP+ system at all, I am just saying that we have not been successful at planning in that way. Yet. I am not saying I will not try this again.
For us, I could tell that the DAS was not working and it wasn't helping to get a return time if we did not use it or did not use it until two days later (though I was happy that we were able to use that one! Thanks!). So that's why we had just three times written in over 10 days. We enjoyed walking around, shopping in stores, going into walking exhibits, riding trains, etc. Our best day was when there were thunderstorms and the crowds at MK were low.
The biggest problem for us was not knowing if there was an alternate entrance to use, even with the FP+ or the DAS return time. This may just be a learning curve for me and is something I am going to investigate. I asked at IASW and was told that there was no where else to wait where it was quiet and that we had to walk through the regular line. The CM told me that they did not offer an alternative waiting area anymore. This created a lot of anxiety and screaming and the wait time was not an issue, just the sensory overload from the hustle and bustle of the queue was the problem. This also happened at HM (though on our second try during the rain, we did find out how to skip the elevator huddle), and on POTC (two aborted tries there due to the queue being quite noisy and anxiety-provoking).
Again, I am not saying that Disney owes me ANYTHING at all. I just need to figure out how to use the system, if there would have been somewhere else to wait (walking around and waiting is often overstimulating versus having a quiet place to chill), whether it can work for us, etc.
Every family is different. I can see how the DAS would be fine for many.