Changing employer while pending I-485

If I-140 was approved, I-485 was filed more than 6 months ago and the 
Employment Authorization Document and the Advance Parole have been issued:

- What is the risk of changing the current job and leaving the I-140 
sponsor employer?

- What does “same or similar job” mean?

- Would it be considered “same or similar” to join a graduate school 
through a graduate assistantship?

- If the employer revokes the approved I-140, will it possible to renew 
the Employment Authorization Document and the Advance Parole?

Your help will be appreciated so much.
Thanks in advance!