It’s all take it or leave it. Occasionally, if someone cancels a MaxPass in advance of their window, you’ll be able to see an earlier time than what’s available but any of those go quickly & you need to be eligible to pull a FP at that time. If you aren’t eligible and you act quickly enough you can cancel whatever latest selection is preventing you from being eligible & then go back & grab the earlier time you saw. But if you’re used to WDW’s modify trick, it seems like an extra unnecessary step.
My experience with MaxPass is limited, but on our first try of it we were at DCA at night & looking for a Big Thunder FP for DL but it was more than hour out & I didn’t know how late the kids would be up for it so I didn’t grab it. I checked again 15 or so min. later & lo & behold there was Big Thunder availability for just 15 min. in the future so I nabbed them. I’ve heard others report that they see it as well, but it would require frequent checking of the app to stay on top of what’s being currently distributed for what times.