Can I switch from a Non-B visa to a tourist visa while remaining in Thailand?

You should be already on an extension based on employment, not the stamp directly from a Non-B visa. Unfortunately, no tourist visa/extension inside Thailand is available for you. You need to cancel your extension (and work permit) officially at the end of your current extension or when you finish working. Unless you can apply for any Non-O based extension immediately, you have to leave.

You need to meet the requirements entering Thailand whatever they would be when you enter. There is no specific rule by immigration to re-enter in a short period such as you have to be out of Thailand xx days.