Porting priority date from Employer - A to Employer - B

[engalveedu]

Hi,

I have been on H1B since 10/2005 with Employer - A (but laid off in 10/2011) - got my I140 approved with priority date of 06/2008 through Employer - A.

Moved on to Employer - B and has H1 petition approved but Employer - B is not willing to do GC.

My question is : if Employer - B or Employer - C (prospectively a new employer) is willing to start my GC - what should I have to do specifically for porting my priority date obtained from the GC process through Employer - A ?

Any letters or any documents that I need to obtain from Employer - A ?

Thanks.