Until relatively recently, PayPal provided no protection at all for anything they considered to be a "virtual product" such as an essay delivered via email. They do now, but they won't consider the academic "quality" of the essay. They will, however, consider disputes if it was written in the wrong language, or in obviously incoherent English, or on a totally different topic than was ordered.
How can Paypal deal with such a dispute when the resolution to the case is not a change of item but a revision?
By simply referring to the TOS for the project that should be in your invoice and/or in the note attached to any payment that you accept.
Only the initial stage is automated. Once you escalate to a claim, every dispute is handled and the outcome determined by PayPal representatives (unless the seller fails to respond altogether).
...Paypal normally sides with the buyer in disputes...
That's not true, even on eBay, where there's a tangible product, let alone intangible products delivered by email.