I fall in below category " Upgraded EB3 to EB2 ( If you had approved EB3 i140 earlier with employer A and then upgraded to EB2 with a new employer B. )
File EB3 i485 without joining EB2 employer?
I believe the employer A still has the EB-3 position and has not withdrawn the approved I-140. You mentioned I can file I485 with supplement J signed by the EB-3 employer A without filing a new I-140 and convert to EB3.
Does this means I can do this without trasferring my H1B or joining Employer A?
does employers agree and entertains these kind of requests for filing supplement J?
Any employer is doing business and they won’t simply file your EB3 i485 without expecting nothing in return.
So, I do not think your employer would agree to do it unless you join them and help them make some money by working for them.
EB3 emloyer can file i485 without employee on payroll
If your EB3 employer has any change of heart by any means, then yes, they can file i485 without you working for them but they will have to prove that the job exists and you will join to work for them.
As I already answered, employer can file i485 with supplement J without you joining them.
It is a good faith declaration on the basis of understanding that you will join the employer once i485 is approved.
The chances are higher for an RFE if you do not join but it is legally allowed.
Also, you should join the employer if i485 is finally approved to not attract any adverse action from USCIS.