This may answer your question:
If you have a valid OPT status until March. You will have 60 days grace period, and if your employer file a H1B petition within the grace period, your OPT grace period will be extended while the H1B is pending. If USCIS approve the H1B, you can continue to stay until 9/30/2013. The problem is that your OPT employment authorization will not be extended in your case.
This answer is of a general nature and should not be relied upon as final, nor is it intended as legal advice. Consult with a qualified attorney before making any legal decisions. Gen Kimura, (832) 247-6932.
If the H-1B is filed within the 60 day grace period, you can continue to stay in the US until USCIS makes it decision. The problem here is that you will not be allowed to work.
Your employer needs to retain an experienced immigration lawyer, whether myself or one of my colleagues, to review all the facts, advise them, and handle the case.
J Charles Ferrari Eng & Nishimura 213.622.2255 The statement above is general in nature and does not constitute legal advice, as not all the facts are known. You should retain an attorney to review all the facts specific to your case in order to receive advise specific to your case. The statement above does not create an attorney/client relationship. Answers on Avvo can only be general ones, as specific answers would require knowledge of all the facts. As such, they may or may not apply to the question.