I made reservations last night for me and my wife and the updated Elite Platinum benefits were available for her (free carry on and checked bag, and free seat assignment (non stretch)).
I looked at another existing booking (purchased December 2023) going out later this month (just for me) and I got free stretch seating and a free checked bag.
Today, I got on the website and I'm an "Elite Platinum" now. So Frontier followed through on the Elite 50K upgrades!