I-140:
After I-140 is approved, one can change jobs in the same skillset, BUT one would need visa sponsership (and that means get a H1B visa). For getting greencard after that, one would need to restart by applying for fresh labor certification/I-140/I-485.

I-485:
After one has applied for I-485, one can get EAD after a few months, which can help one switch jobs without authorization.
After I-485 is approved (which is when one gets a green card), one becomes a permanent resident and do anything. A work visa is not needed on green card.

This is just my opinion (I am in the middle of the process myself).
So, Please seek a immigration lawyer for your specific issue.