converting the database does not alter the original file. Why can't you open the original copy using FileMaker 11?
Account names and passwords must both be entered to match to an account name and password defined in manage security for that file. It's possible that you specified a different user name when you installed Filemaker 12. FileMaker automatically enters the user name as the default account name when it displays the password dialog. Thus, it may, in fileMaker 12, be entering the wrong name for the account name. You may need to change this user name to the account name defined in the file before it was converted to .fmp12 format.