Yes, use that A#. But, don't get freaked out if they change it later on.
PROFESSOR OF IMMIGRATION LAW for over 10 years -- This blog posting is offered for informational purposes only. It does not constitute an attorney-client relationship. Also, keep in mind that this is an INTERNET BLOG. You should not rely on anything you read here to make decisions which impact on your life. Meet with an attorney, via Skype, or in person, to obtain competent personal and professional guidance.
Once you are issued an A number, that should be your A number for all intents and purposes.
Please note that this posting does not constitute legal advice or create any attorney-client relationship with the inquirer. Avvo is a useful forum to obtain general information on legal issues, but is not a substitute for direct, confidential consultation with an attorney in any legal matter.