There are two different FAQ about Thailand Pass. It appears that the first one said that changes can't be done, and the second says flight changes within 72 hours are ok.
First from October:
คำถามที่พบบ่อยเกี่ยวกับระบบ Thailand Pass (สถานะ ณ วันที่ ๓๑ ต.ค. ๒๕๖๔) - กระทรวงการต่างประเทศ (mfa.go.th)
Second from December:
Thailand Pass FAQs - Department of Consular Affairs Ministry of Foreign Affairs (mfa.go.th)
Can I change my travel itinerary once I have registered and received my QR code?
- Yes. However, flight changes are permitted within 72 hours after the original flight time registered in the Thailand Pass without the need to apply for a new Thailand Pass QR Code, provided that all other required documents remain valid.
Unfortunately they added that vague sentence at the end "provided that all other required documents remain valid." Which brings up the question in my situation of I had to change the hotel reservations dates, so the original documents are no longer valid. The gate agents were leaning toward I need to reapply, which would be fine, except I'm going back to the airport in less than 12 hours. One told me to call the consulate using what I think is a number intended for airlines, and of course the guy there said my situation was ok, and then I said, I don't know if it is posted anywhere, but if they called you tomorrow, could you verify? And he said it is on the Thailnd Pass website. So I searched and found the second FAQ. There is a .pdf download at the bottom that repeats what is on the page.