I 140 approved by previous employer

Hi , I have an in140 approved from my previous employer and my priority date is March 2015

I changed my employer 6 months ago( it was better move for me) and they just started perm process and it may take another 18 months before I get my perm Approved.

If in any case the visa bulletin in the next couple of months or in October bulletin , DOF becomes current for me , can I go and ask my previous employer to sign the supplement J for my i485 ?

My employer is not a desi consultancy but it is a big American staffing company. So do they generally agree with my I 485 process ?