Try using the "reply" button at the bottom of messages, so Itch lets that person know that he has a reply.
Back to the topic. Don't change your current game page, create a new one that is free (PWYW) and leave the other as "premium", Itch allows you to have 2 versions of the same game, as long as one is free and the other is paid (premium content).
In your second part, you put a sale with a discount and with the condition that they must have the "premium" version of the first game.
That would be enough. But if you want to be a little more elegant, you can hide the premium version, that way, anyone who searches for your game will always reach the PWYW version, and on that page, you create a link with all the information and advertising of the benefits of supporting you by buying the "premium" version.