I found the answer in CIS Interoffice Memorandum HQPRD 70/6.2.8-P (dated 12/27/2005) (from VB):
Question 11. When is an I-140 no longer valid for porting purposes?
Answer: An I-140 is no longer valid for porting purposes when:
A. an I-140 is withdrawn before the alien's I-485 has been pending 180 days, or
B. an I-140 is denied or revoked at any time except when it is revoked based on a withdrawal that was ted after an I-485 has been pending for 180 days.
According to this statement, it seems that the former employer can't revoke the approved 140 if 485 has been pending for 180 days.
Is that correct ?
Question 11. When is an I-140 no longer valid for porting purposes?
Answer: An I-140 is no longer valid for porting purposes when:
A. an I-140 is withdrawn before the alien's I-485 has been pending 180 days, or
B. an I-140 is denied or revoked at any time except when it is revoked based on a withdrawal that was ted after an I-485 has been pending for 180 days.
According to this statement, it seems that the former employer can't revoke the approved 140 if 485 has been pending for 180 days.
Is that correct ?