Okay, so I'm back from an afternoon at AK and hopefully tested everything I was supposed to. I had 3 FP booked in DH & my name (ITTBAB at 8am which I didn't feel bad "wasting", Everest at 2 and FOP at 3:25), and Na'vi River Journey booked in 2 of my sons' names (AP's) for 4:15 along with Primeval Whirl at 2:30. Here's what I tested and found out:
1. As soon as I came in I booked a SDFP for someone previously associated with my MDE using their old card with no ticket remaining. I then (on my phone/app) used the Change Party function and had no problem (!!) changing the party from that person to my son's AP (since he had valid admission, although he wasn't in the park). No guarantee this will work after the 6th but could be a game changer if it still does and the "check point" is at the ride as we've heard and not the kiosk.
2. I waited until I was well in the window (about 3:10) for the Primeval Whirl FPP at 2:30 and was able to change the party to 2 other folks in my MDE with AP's without issue. Once I'd changed them, I went ahead and canceled them (not a fan of that ride).

But even using the app, I was able to change within a window, which I thought I had done in the past.
3. As far as tiering for AK, I had let the ITTBAB in our "real" names expire (FP #1), used the second one at Everest when we got in the park (FP #2), and we then went to FOP (FP #3). While we were waiting in the long line there (but before we had scanned the second time), I went in and used Change Party to transfer Navi River Journey from my sons' names to our names. We rode that as well without issue using our "real" bands/names. So apparently it's fine to still let one Tier 2 expire (and use the other and the first Tier 1) and then be able to book another Tier 1, and AK works like the other Tiered parks do in that regard.
And of course as we were leaving Pandora I had a woman come up to me and offer me a rider swap for 3 people for FOP! I thanked her very nicely but told her I was local and had ridden several times already and she should offer it to someone who could use it more.

Hope someone got to use it! Anyway, I'll be back there tomorrow so if there was anything I forgot, let me know. Realize this is all subject to change in another week!