How do I cancel my work permit and non-B visa in Bangkok, and what are my options for applying for a new visa?

1. -> Go to the labor office bringing the resignation documents from your employer to cancel your work permit, then go to your immigration office with a work permit cancelation receipt to cancel your extension.

If you are working for the company for more than 3 months, you are on an extension based on employment, not on a Non-B visa anymore.

If you are still on the first 90days from Non-B, you don't have to (cannot) cancel your stamp. You can stay until the stamp runs out.

2. -> You can try applying. But if it's approved or not is at the discretion of the officer. Be prepared with a Plan B.

3. -> That depends on your current visa/extension situation, new employer, and the immigration office in the new province. Not all employers can (or want to) provide the documents for you to apply in-country, you need to talk with your new employer first.

And you cannot rely on Covid extension in advance.

Be aware, that without all details (your nationality, your visa/extension history, employer, old and new immigration office, and more) being exactly the same, someone else's experience may or may not be applied to you.