What will happen if i change my employer after I-140 approval?

Hi,

My green card has been applied in EB2 category. What will happen if I change my Employer after my I-140 is approved by the USCIS. Can my approved petition be carried further or my new employer has to start the process all over again. If the process has to be started all over again then what wil happen if my 6 years in US are completed before the approval of petition by 2nd employer.

Thanks