Change employer while I-140 under process

[mkey_legal]

Hi all

I am H1B visa with a consulting company and my current employer has filed my I-140 3 weeks before under EB2 category. Now I am getting an offer from a client itself. I have a good relation with my consulting company, but I am tired of moving for every job so I am trying to take a permanent Job. So if I ask my current employer to move my I-140 to premium processing and once I get the approval can I take up the new opportunity or is there any rule that I have to work for my current employer for 6 months or so after I get my I-140 approval. Or is there any rule that even after I get GC thru my new employer I have to work for 6 months with my old employer after I get the GC.