1. You can go from tourist visa to Non-Imm ED visa inside the country (or tourist visa to Non-Imm B visa). But it is not possible to go from Non-Imm ED to Non-Imm B visa (in other words, change Non-Imm category) inside the country.
2. If you get an ED visa, you are legally obligated to take a minimum number of hours of lessons each week.
3. Right now, I believe getting an ED visa for a private language/fighting school inside the country isn't even possible (they stopped in 2020).
The 7-day stamp to leave the country is a good option, but wouldn't switching to a COVID extension be better? Allowing OP a full 60 days to prepare flight and so on to leave the country?
Do you mean cancel the extension AHEAD of time, or rather talking to the employer to so that the extension is cancelled two or three weeks AFTER the end of employment (allowing a few weeks to prepare to leave)?
I don't see why anyone would want to cancel their visa in advance... ?