I am currently working on 3 yr H1B which is expiring on this October 10th. My Green card was already filed by my employer and right now I am in the stage where my I-140 is approved (priority date October 2008).

I am planning to go back for studying on F1. The classes start in the last week of August 2009. I will be studying in the school parallely while working on H1B until October 10th. After October 10th, I will be leaving my job and studying full time.

1.Since I want to stay in USA, should I apply for Change of Status (COS) with I-539 form from H1 to F1, even though my I-140 is already approved? Does I-140 create problems for COS to F1?
2.What should be the start date on I-20 (Aug' last week or Oct 10th)?
3.Does going back to F1 cause problems to my current GC process?
Thanks.