Be very careful with that. If you have a thai speaking friend, ask his/her help. I did it the other way around and it was a mess. When they say "easy" it means they don't know 😀
I can only write what happened to me. My school cancelled my work permit and told me 5 days later. When I went to immigration I had to pay 5 days fine regardless of my 90 days stamp (still had 26 days to go).
Hello Emma! They just gave me the cancelled work permit (one week after they did it). I showed it to immigration with a filled form of my cancellation of my visa. I directly applied for Covid extension and paid overstay fee thanks to the old school. The school have to give you at least the old work permit.
Make sure that you go on the exact date of your work visa cancellation. The school I worked at cancelled my work visa without telling me. Had to pay overstay, but could apply for a Covid Visa.