回複：真的會很麻煩嗎？換工作前我們征詢過律師，他說是可以的，因為

Yes, so if the 前公司出具的employment letter promise of rehire and future offer, you don't have to work for 前公司 before you get GC.
If you intent to use AC21, you must satisfy IO that the emploument letter AT THE TIME of you filed I-485 stated the true intent, because you left the sponsor BEFORE you filed I-485 with 前公司 letter.