Hey there. Good question and I'm not fully sure on the answer but i can share my experience. I changed jobs during my extension period. At the time, I did not have my new green card but when the employer asked to provide information of my status I only uploaded my NYC Driving License and my SSN. This was enough information for them to accept and not ask questions around my GC. I've heard of stories in the past that in some cases you may be asked to upload the extension letter and the expired GC as proof. If the employer says they need a valid GC you should push back and say this is enough proof for I-9 verification.
