Hi, I have a valid H-1B visa and have been working in USA for one year. Recently I got an offer from another employer. They have filed H-1B visa transfer for me. USCIS received my petition on Thursday July 10th. My new job starts from 21st July 2014 and 18th July 2014 is my last working day in my current company. I have yet not received my receipt number. Do I need to wait for my receipt number or can i join my new company on 21st July 2014?