So, for WDW, I know you can change your room rate after you have purchased it, if a new discount comes out. What about Universal?
Can I buy a room at the standard rate now… and then if AP rates come out later, can I apply it?
I’ll be buying flights tomorrow for a Universal only trip in May. I’m tempted to just buy the standard rate for the hotel too. Our dates will be around Memorial Day, May 25-29, so I don’t want the basic rooms to sell out. I’d hate to miss out on a nice AP discount though.
Yes you can. That’s what I was told anyway. There didn’t end up being a discount for my dates.
Yes. I booked regular rate, then when 2020 military rates came out, they updated the rate only (kept same reservation number). I assume it works the same for other discount rates.
Did you have to get it changed over the phone then? Or should I be able to “modify” my reservation on the website?
You have to call for military rate, so I’m not sure if it works online for other rates.
Gotcha, well thank you so much for the info! 
Yes, you can cancel a reservation a week prior to the start of the stay so if you find a better rate book it then cancel the other reservation. I’ve done this a few times with no issues.