I've only found this: https://itch.io/t/987840/what-happens-if-i-increase-the-price
Maybe that's what happened, with the "set minimum price" option? In that case yes, everyone who has already bought the game would have to pay 15$.
But the other option would make it so whoever already has the game won't pay 5$ more, so i'd say it wouldn't be ideal either.