You are (mostly) incorrect. AC21 portability (as it's called) allows for an employee to maintain their pending immigration status if they have filed an I-485 a

You are (mostly) incorrect. AC21 portability (as it's called) allows for an employee to maintain their pending immigration status if they have filed an I-485 and it has been pending for at least 180 days. Thing is, the I-485 is the last 10% of the process. The biggest hurdle is getting the Labor Certification, which is the part that can (and does) take years. So an employee technically has H1B portability under the AC21 provisions but they must start all over again if they aren't in the last 5-10% of the immigration process.