By using your argument, why people cannot change job before I-140 is approved? They can absolutely argue that they will return to the job when GC is approved. As far as employer is happy, the employer will not revoke your application, and what risk you will get?? In both cases, there is no difference in the risk. As far as you keep good relationship with your employer, you can totally forget about the AC21. BECAUSE, you can always say you will go back to the old company.