Demo accounts are not in any way linked to live accounts. If you would like to continue having a demo account after you have a live account, the logins will be completely different. Also, demo accounts expire after 30 days.
You can register for a new demo account at the link below:
http://www.forexmicrolot.com/open-free-micro.jsp
You will need to select the Connection "Demo" whenever you want to sign in to your demo account.
You can access your account while it is still being processed, but you will not be able to trade on the account until your account processing is complete.
Regarding account number/username, I will certainly pass along your concerns regarding the perceived confusion.
We appreciate your feedback.
-Brad