The change is not retroactive; that's stated in the rules and has been discussed in other posts.
If someone purchased your product through Itch, they've already paid Itch, and the only way to withdraw that money is for Itch to pay you through the normal withdrawal process. Only new payments made after the change will go directly to you, because that person will be redirected to your PayPal account at the time of purchase, not to Itch.