[Tsuwabuki]

JET has more control over your visa because they are a quasi-government program (discussion of CLAIR, bidding practices, etc, are outside the scope of this post) and usually only gives you thirty days before they revoke sponsorship. I have my doubts he was on JET.

My guess is that he was an eikaiwa (conversation school) or dispatch ALT teacher. Legally, an employee may quit at will (or may be terminated at will), and usually how notice is given is covered in the contract. He probably just spoke to his boss and explained it wasn't working out after making sure his sponsorship for change of work industry was discussed with his old (and current) boss. He would have then just taken the paperwork to immigration and changed his visa type (or may not even have had to, actually. My visa allows me to do a lot more than just teach, which is what I do with it. If I had an instructor visa, I would be locked in).

As for finding teaching jobs, JapanForum (while a great forum for Japanese culture in general) is not a very good place. Your best bets would be GaijinPot, Dave's ESL Cafe, the Ohayo Sensei newsletter, and googling. A lot.

Note, we are past the hiring peak, and the market is relatively flooded right now. This doesn't mean you can't find a position, just that it is not as easy it has been in the past, and that overseas individuals without visas and previous experience have a harder time than visa holders in Japan who already have experience (and there are a lot of those looking for work already).