GC got approved before filing AC 21

[arudiarun]

Hi, Recenty by GC got approved and before that I switched job from sponsoring company. When we were in the process of filing AC-21, my case was approved. Now, when I spoke to company's attorney, she told we can get the documentation done and keep it ready in case if I need it in future when I apply naturalization. She also mentioned, AC-21 can't be applied at this time since the GC has been approved. Does any one come across this scenario? will I get into any issues later? How does USCIS knows that I changed the company before I got my GC? Thanks.