The new AC21 memo (ref: first thread) clearly states that if the I-485 has been unadjudicated (and I-140 approved + unrevoked) for 180 days, then you can change jobs no problemo.
Even if the employer tries to revoke the I-140 after 180 days, INS will not stop the I-1485 process. If you change your job to a similar position and inform INS, you will be just fine.
I hope by changing careers you will not change the job description in the LC (i.e. software engineer to Stock Analyst) - that kind of career change is not allowed by AC21.