I appreciate that you're honestly talking about your experience and trying to do the right thing. As you can see, the use of AI is a fraught topic, and I think people should acknowledge that you are trying to do the right thing by supporting artists.
To answer your question-
I would invest in new art for a new product. As you wrote, you are updating the product in other ways- from art, to formatting, to some of the rules. That makes it ... a new product. The other people received an old product, and either paid (or didn't).
If they want the new product, then they can pay for the new product. If anything, think of the old product as a "beta version" with "placeholder art" and this is your actual, for-sale product.
Now, if you can, and if you are feeling generous, you might want to provide a discount on the new product to people who paid you money for the "beta version." That would be a nice thing to do, but it's certainly not required. The margins in the industry are small, and you probably won't be making money no matter what (right?). In the alternative, maybe you can offer some "bonus" thing- something small, like an extra, to the people who paid for the beta version, as a thank you. But again, you are not obligated to do so, although it would be good customer service and might make them more likely to pay for the new product.