You can, but it does not work very well at all. I had my first go at it Saturday.
I got to AK and they said they had nothing for Safari, Dino, or EE. So I settled for a Primeval Whirl FP that was open immediately. Not great, but it had a 30 minute wait since it was early afternoon so it was better than nothing. When I got over there, I saw the wait for Dino was only 20 minutes for some reason so I decided to hop in it first.
While in line I thought I'd see if I could change my FP to something else. I clicked the experience I wanted to change (PW) and then it showed me options. We were only going to be in the park for 2-3 hours so I needed something soon. I saw Everest was an option now so I clicked it. I was never shown times or anything. I just simply changed my FP to EE, only it was for much later in the day than I wanted. I was never shown times or even asked "are you sure you want to make these changes" like I had been when pre-booking my first 3.
What was worse, was I couldn't even get my PW FP back. It was now giving me one for 3:00 instead of 1:20 like I originally had. Fortunately the CMs at the FP booth were able to get me a FP for Everest at an earlier time that I could use and I rode standby for PW. So in the end, it worked out better, but it was still very frustrating when it happened. After that, I didn't both trying to change it on my phone.