回复：Can I-140 be withdrawn after it is approved?

your current employer can apply to withdraw the I140, since this I140 belongs to them. HOWEVER, it wouldn't affect you. Because the I140 was approved and your I485 has been pending over 6 month, USCIS wouldn't accept their withdrawing application.

Worse case for you, you may be required an interview later. Pls note, after you switch your employer, you need to file an AC21 appeal to USCIS to state that you've changed your employer.