Even after crossing 180 days while pending AOS an applicant CAN change the employer by exercising AC-21 rule, what difference it makes to move on another employer after getting GC in hand??? This is very much debated issue, and no one has reached any proper conclusion as USCIS doesn't adhere any rules regarding the stay within the sponsoring employer. While filing Citizenship, USCIS MAY ask to provide 5 years of tax return. It would be wise to file for Citizenship after 6 years in that case.