My business model on itch is that I release stories for a small fee, then on the Monday on or after the day six weeks later I set the story to PWYW.

One of said stories comes in two parts, the second of which is yet to be released. I have promised that paid owners of the first part will receive a discount on the second part when it releases, but have realised since that this is not actually straightforward to implement in itch as the interface does not distinguish between owners who have paid a certain amount and those who haven't on a PWYW item, at least not in a list I am allowed to use for this purpose. 

What I want is to be able to distribute a discount on the second chapter only to people who have paid at least the list price for the first, either before it became PWYW, or afterwards when that became the recommended tip. How might I best do this?

One way I think this could be achieved is that I could send an email to all owners asking that they contact me via their attached itch email for their discount, and then cross-reference the addresses of any replies I get with emails on my payments sheet to determine who should get a reply with the discount. But this is inelegant, and would only even work because of the small number of people who actually own the story and the brief window in which the deal would be available. On a more successful item it would be infeasible. Is there another way?

In Itch, with the PWYW model, you have no way to limit the minimum purchase price.
Under PWYW if a person donates 1 dollar, for Itch, that person has purchased your project and has full access to the project.

The "automatic" alternatives that I think could be valid

A)
Create a discount coupon and place it as a FILE with a minimum price within your PWYW project.

If someone donates less than the price of that file, they will own the project, but they will not be able to download that file and therefore will not be able to access the coupon. (People who have already purchased your project will be able to access the file if the purchase price is equal to or greater than the price you set for the file.)

Con: I think the coupon has a lifespan, but I'm not sure.
Also, if someone shares the coupon on the internet, anyone who copies the coupon URL will be able to use it.

B)
The other alternative that occurs to me, which I see as more feasible, is that you generate a second page with the PWYW project and leave the first one only for payment. You can hide it so that it is not indexed and place a link on your PWYW page explaining that if someone wants to donate X amount to you, they can do so by purchasing the old version so that they can continue buying the new versions at a discount.