I'm working with Company-A in US for last 6.5 yrs, my I-140 was approved in May-2009 in EB3 and current H1B ext is valid till Dec-2012.
I got another offer from Company-B, and they agreed to start new GC process in EB2.

Below are my questions:

A) Should there be an exact match between job descriptions in company-A (for which I-140 was approved) and in Company-B For:
(a) transfering H1B ?
(b) Porting my current GC priority date (company-B agreed to file in EB2) ?

B) What are my options to continue work in USA If:
a) If I loose job in Company-B before they start my GC processing
b) If I loose job in company-B after they start my EB2 GC process

C) Can I use the I-140 from either company-A or company-B to apply H1B transfer through Company-C to continue, work in USA.