The change of employer when the Form I-140 petition has not yet been approved by USCIS is risky. One of the key problems is the possibility of a Request For Evidence (RFE) on the Form I-140 petition which may be issued by USCIS. 

When the Form I-140 is reviewed, the USCIS can issue an Request For Evidence (RFE). Since the Form I-140 is filed by the employer, under current practices, the RFE is sent to the employer. Thus, even if the RFE addresses matters that involve the alien beneficiary, such as education and experience, it will be sent to the employer. If the employer does not respond, or responds stating the alien beneficiary is no longer employed by the petitioner, the Form I-140 petition will likely be denied.