You cannot hold non-immigrant status and be outside of the United States, and your L-1 visa is a non-immigrant visa. Thus,
when an applicant for a change of nonimmigrant status leaves the United States while that application remains pending, the application is automatically deemed abandoned and becomes void.
So as I have already leave US and returned back to US while COS from L1B to H1B is pending, Will it void my complete H1B petition or just COS?
I am not sure what travel dates you have in mind, but if you want the COS to proceed you will need to wait until that process is completed before you leave the U.S. If you leave the U.S. after approval, you would get your visa stamped upon reentry.
I don't have any travel date for now I am just waiting for approval and after that I will go out of US for visa stamping at consulate. Is it required to leave US before 1st-Oct to avoid confusion of my status after approval of H1. As my cos might get abandoned.
Thanks in advance for your response.!