changing employers in midst of GC

[QVS]

Hello,

My H1B started in Oct 2007 and the 6 terms are going to expire in Sept 2013. Last year, I applied for GC with my firm in EB3. My Labor stage is complete and I have got a PD. Currently my I140 has been applied for with USCIS.

-In what stag eshould my GC petition be to enable me to renew my H1B visa status?

-In case I need to change my employer now, does it give me enough time to restart my GC application with the new employer so that my H1B visa can be renewed in Sept 2013?

-What would be the earliest that the new employer would need to apply for my GC so that my H1B visa can be renewed?

Thanks in advance for any assistance.