I-140 Approved (PD: Jan12) ; H1B valid until Mar 17 ; Can i change employer ?

Q, All,

I have an unique situation - Appreciate if you could help.

My current employer (A) has filed GC.
GC Status:
Category - EB2
Current Status - I-140 Approved
Priority Date - Jan 2012
(p.s:. My PERM went into Audit and got cleared)

H1B status -
6 years completed
Currently on 3 year extension based on I-140.
H1B extension is valid until Mar 2017

However, I got an offer from my previous employer (B).
They've promised to initiate GC process (starting from scratch) and port my original priority date.

Questions:
1. Can my previous employer (B) do H1B transfer which is based on I-140 of my current employer (A) ?
2. Can I negotiate with my previous employer (B) to initiate GC in EB1 category as i've already worked with their offshore country ?
3. Even if they file in EB2, can they really port my original priority date (Jan 12)?

My thinking was, anyways the PD for EB2 stands at Jun 2004 as of now.
It might take an year or two to get my PERM and I-140 approved with my employer (B).
Most likely the PD for EB2 may get current by the year 2016 and i'll have no wait to file I-485 and get EID and GC.

What are the risks i should anticipate and is it worth to switch job ?

Thanks in advance.

Thanks,
Neo