Yes, you'll be able to make your first
DAS selection on the app once you scan into the park. Sometimes it can take a few minutes after you scan in for the DAS info to show up in the app though. This happened to us both in the morning and returning to the park.
For many rides you'll enter through the lightning lane and you'll need to scan your DAS holder first, then the rest of your party. Once you are scanned in to a ride, you'll can immediately book your next DAS.
We had some trickiness with the DAS sometimes not clearing after we scanned in, preventing us from making a new selection. You can always cancel a DAS that hasn't been redeemed or go to a guest services kiosk and have them help. Sometimes weird things happened where one party member would not clear and we need help to clear it and add it to the return we already booked.
If you are parking hopping you won't be able to book the other park on the app. You can always to guest services to book though on your way to park hop, take a hotel break, etc.
If you the ride you have a DAS time for breaks down, you may be given a multi-experience pass to use another time. This happened to us on Soarin' our first morning (pre-booked DAS) and then we used it at the end of our day to ride Soarin' again (could have used on other rides, but my DS wanted to do Soarin').
We had a hard time using our pre-booked DAS times. We often just weren't in those parts of the parks when the time came. But no big deal if you don't use them.
Have fun!
