It's not practical to COS to B-2 - 回复：请教PJIANG：出境＂激活＂签证

You are face a gap of Jul 8 to Sep 30.

1) if you file I-539 close to Jul 8, and B-2 is granted most likely after the approval of H-1, you learn "Last Action Rule" here
http://blog.wenxuecity.com/blogview.php?date=200709&postID=16851#LastActionRule

2) if you file I-539 earlier, and it is approved before expiry of OPT, you need to stop working. It is not guaranteed H-1 approval notice comes with I-94, though.