you should be able to, but with condition

If you want to change job before 180 days of your Form I-485 pending, you should be able to, but with condition:

1) If your went through the PERM Labor Certification before, your new job should be close to what described in your PERM Labor Certification.

2) If your did not go through the PERM Labor Certification (such as EB1 or NIW), the new job should be in the same filed as described in the Form I-140.

AC21 portability is generally available to an individual who is the beneficiary of an approved I-140 and whose I-485 has been pending at least 180 days. The benefits of AC21 portability are available to any otherwise qualified individual, even if there is not an available visa number for his/her case.

Also see:
www.greencardapply.com/news/news07/news07_1230.htm
www.greencardapply.com/i140/i140_changejob.htm