L1A > H1B > COS Approved / Start Date Oct 2012. Can I request change COS Start Date?

[s1111]

Hello,

I have L1A from current employer A expiring June 2013.

I recently got approval of H1B for emplyer B with COS approved with start date of October 2012 expiring Sep 2015.

L1A - GC process is much faster and my employer A may be considering filing my petition under EB1C, is it possible to extend COS date of H1B to some time later next year, so I may possibly avoid problems with my GC processing if employer A agrees to file. What issues may arise in such a case?

If I do switch from Employer A to B (L1> H1), can employer A's petition and any subsequent filings after COS still be valid? Or do I have to remain employed with employer A?

Your comments are highly appreciated.

Thanks

S