My employer sponsored I-140 petition has been approved, and I have not filed the I-485 application, because of no visa number available in my country. Now due to the continuous economics downturn, it is very possible that my employer may layoff employees, and may include engineers like me. My question is that can I begin search for new job, or change employer if possible?

With economics downturn and unemployment continuous increasing, some alien workers with Green Card application in process may begin the job search. Depending on the stage of the alien's Green Card process, the job changing or layoff can result in denial of I-140 by the USCIS, as the I-140 petition and I-485 application require that both the petitioning employer and the alien beneficiary have their intent to work for the employer. 

An alien's activities looking for other employers or changing job is the evidence of the alien beneficiary's intent not to work for the petitioning employer. If the I-140 petition or concurrently filed I-140/I-485 has not reach 180 days to invoke AC 21 I-140 portability to a new employment, such I-140 may be denied, or if it already has been approved, such approved petition may be revoked.