First, I really doubt if your employer will inform INS. But even if they do, you can start working with your USPS delivery confirmtion card of your new application (as my friend did.) But I recommend you resign at least after your lawyer has sent out the application because things might change before that. Then after that you will get offical receipt in a couple of weeks. In my case, my new employer did not ask me to give notice before they got the receipt. Hope this helps.