Looks like Matthew Oh thought about it a bit more and updated his FAQ:
Q45: I am an Indian and my labor certification was approved in May 2004. I immediately filed concurrent I-140/I-485 which are currently pending. My I 140 petition was approved in September 2004. In June 2005, I changed the employment of a same occupational classification. I have just learned that my former employer had withdrawn the I-140 petition in October 2004. I know I am O.K. as far as pending I-485 application is concerned. However, I am terrified that my priority date was lost because of the revocation. What would be my priority date at this point? A: Under the rule of the priority date retention, once the approved I-140 is revoked, the alien beneficiary does not retain the priority date. The withdrawal of I-140 petition constitutes an "automatic" revocation under the immigration statute and the revocation takes effect upon receipt of withdrawal and agency notice. It appears that this raises a serious question of the effect of employer's withdrawal of the approved I-140 petition after 180 days of I-485 filing. This question remains a grey area in the interpretation of the AC 21 Section 106(c). Relevant to this question is the legislative intent of Section 106(c) of AC 21 as related to the question of whether Section 106(c) of AC 21 overrides the immigration regulation which provides that the alien beneficiary does not retain the priority date if the petition is withdrawn and revoked. It should be kept in mind that Section 106(c) provides porting of I-140 petition and not I-485 application. Again, it is not I-485 which is ported but it is I-140 petition which is ported. Under this provision, I-485 is not affected when I-140 petition is ported and the law must assume that the ported I-140 petition remains valid. This theory is supported by the current policy of USCIS which is enunciated in the Yates Memorandum which states that the I-140 becomes invalid in one of the two situations: (1) Either I-140 is withdrawn before the alien beneficiary's I-485 has been pending 180 days; or (2) the I-140 is denied or revoked "at any time" except when it is revoked based on a withdrawal that was submitted after the I-485 application has been pending for more than 180 days. In other part of the Memorandum, Mr. Yates also stated that priority date was not affected by porting under Section 106(c) of the AC 21. Yates Memorandum also states that no matter what happened with the I-140 petition, when the alien ported after 185 days of I-485 filing, the USCIS should still go back to the pending (yet to be approved) I-140 petition and adjudicate the pending petition. If they find out it was not approvable at the time of I-140 filing, they were required to deny the pending I-140 petition and related I-485 application. Analogy is found in other section of the AC 21 as related to H-1B porting. When an H-1B alien changes employer who files a new H-1B petition and ports, it is the underlying exiisting valid H-1B petition which is ported. It appears that once the H-1B alien ports, the law assumes that the existing H-1B remains valid for the remaining period of I-94 and it will not be affected by the old employer's withdrawal of the H-1B petition. Different lawyers may have a different view and you should seek legal counsel from your own lawyer. This Q&A is not a law but just an opinion which does not have a legally binding force. You are reminded that you should never act purely in reliance on information on the internet, including this web site. There is a disclaimer in each site.