the 7 days to leave by stamps are given to people who's visa, or extension, is expiring, like lots of business people getting laid off with no warning, or tourists that can't get another extension, but try to apply for one anyway (and is used the most), when their entry is about to expire, not when they intentionally cancel their entry stamp.
don't try to make up situations that weren't mentioned at all by the OP. There was no mention at all that his visa was expired, and he was on his final one year entry. You made that up to fit your agenda.
not at all. He's changing to a type B extension from a type OA entry. He cancels the OA, and immediately applies for his type B. He shouldn't have to leave.
you are correct in that respect, but his isn't expiring, he is cancelling it. There is a big difference concerning the 7 days to leave stamp. If he wants seven more days just wait seven days to cancel it. He's taking a job. His current one year entry isn't expiring. There is a big difference.