You can't have it both ways - get the discount for AP rate when it suits you and also the flexibility that comes with regular rates when it doesn't. Choose one of those and book the appropriate rate - if you want to be able to rebook into a lower rate when it becomes available, book a rate that's flexible (as I said above, I now go for semiflex which is only marginally more expensive and lets me rebook up to 5 days before arrival). That's really all there is to it.

I had a 7-night stay this summer where the rate tanked by 50€ / night, so I lost 350€. Tough luck. But overal I saved much more money with these rates over the years. If I were to ask Hilton to give me those 350€ back, why shouldn't they ask me to give them back everything I ever saved?

(And I'm at 94 nights this year, 89 of which were cash rates, so I'm even more entitled to be entitled.)