Unfortunately that post is now incorrect.
Those who paid 10$ previously do no longer have access to the new 25$ files. They also cannot pay 15$ to get access, they would have to pay the full 25$ again.
I'd gladly pay the difference, but paying the full amount again a month after my previous purchase? That really rubs me the wrong way.
See discussion about this itch problem here: https://itch.io/t/4291978/raising-price-without-locking-previous-buyers-out