any_mor_left and all Teachers:
Thank you for your reply.
When I change employer, a transfer of H-1b has to be filed, and USCIS will know immediately I change employer.
My lawyer said a AC21 has to be filed after I change employer, no matter what my situation of I-140 is. Since in my case, I-140 is still pending, then only when I-140 is approvable, I-485 can be ported.
When should AC21 be invoked? Any experiences in this forum?