I looked up this information on USCIS's website. It is clearly posted that once an I-140 is approved for an alien, the alien can keep the priority date for life.
The only way that the alien can lose this date is if fraud is detected in the case. Employer withdrawing I-140 does NOT affect priority date transfer.
All you need to transfer your priority date is a copy of your approved I-140 document. This is the most powerful piece of documentation in the whole employment-based immigration process.
What this means is that you can take a 10-year break from the US and come back and still have your Priority Date valid. Also, this is valid even if you switch categories, professions or anything else (Example EB-3 to EB-2).
Look on Page 27 of the USCIS Press Release
"(1) Determining the Priority Date. In general, if a petition is supported by an individual labor certification issued by DOL, the priority date is the earliest date upon which the labor certification application was filed with DOL. In those cases where the alienís priority date is established by the filing of the labor certification, once the alienís Form I-140 petition has been approved, the alien beneficiary retains his or her priority date as established by the filing of the labor certification for any future Form I-140 petitions, unless the previously approved Form I-140 petition has been revoked because of fraud or willful misrepresentation. This includes cases where a change of employer has occurred; however, the new employer must obtain a new labor certification if the classification requested requires a labor certification (see the section on successor in interest)."
Let's put this issue to rest now. Lawyers do not always know the right answers!