Job change after I-140 approval but before priority date is current

[dennis_joe14]

I work for Company A. Company A has successfully filed for my PERM, LC, etc and I have an approved i-140 with a priority date of Sept 2009 in EB3. My H1B has now been extended twice based on the approved 140. My present H1 expires in May 2014. I was interviewed by Company B and was given an offer by Company B. Company B is also willing to process my green card application from scratch.

How soon can I start work for Company B? I am assuming Company B will have to go through PERM, LC, get an approval on 140 and then transfer the H1B extension to the new employer - all of which may take 6-12 months and only then could I start working?

Are there any circumstances where I can start with Company B sooner?

Company A has offered to upgrade my application to EB2. Does it make sense to stick around and go through the EB upgrade or move to Company B within EB3. The offer from Company B is considerably better than Company A.