you are correct on ur H1 status from Oct 1st. If ur L1 A VISA stamp is still valid to reenter to US , better option would be go out of US before Oct 1st and come back after Oct 1st. I think, Later ( In jan in ur case ) , can apply for COS from L1 to H1.
If you are L1 I-94 ( expired VISA ) , ideally u should join H1 employer from Oct 1st, I am not sure how u can avoid that. check with attorney.