I would recommend you wait until you have the receipt notice from USCIS before you quit. Once you have the receipt notice, you know everything have been filed and you are free to begin employment with the new company under current immigration procedure.
The information included on this page is a general description of immigration procedure and does constitute legal advice nor does it establish an attorney/client relationship. If you believe this information may assist you with your immigration needs and would like legal guidance, you can contact the Cauley Law Firm, PLLC to learn how the information may apply specifically to your case. I am licensed in the State of North Carolina but can assist with Federal legal procedures questions nationally, which includes the area of immigration.
You may change once the application has been filed. Placing the application in the mail (Fed Ex) is not filed.
Attorney Robert Brown's (former INS Director, 1972-99) reply to your question is general in nature, and does not constitute legal advice as all facts are known to him. For specific advice or representation you should retain an attorney experienced in immigration law. Mr. Brown's reply on AVVO does not create an attorney/client relationship not constitute legal advice.