If your I-140 has been approved and its been more than 180 days since your I-485 has been filed, you may change the employer.  But your employer has right to revoke your I-140 at anytime, since you would be consider as a permanent employee for them.  It all depends how helpful your employeer is. You  may see lots of stories of  I-485 denial cases in different immigration forums.  I am not a lawyer though, please don't consider this as a legal advice.

Good-luck