Hi usafilmschool and welcome to the forum.
Try "Admin" as the username and no password and see if that works, this is the default account details when creating or converting a file.
I hope this helps
Also, converted copies of files with defined passwords can be opened by entering the password as both account name and password. Be aware that passwords in old versions were not case-sensitive while Fmp 10 passwords in the converted file are case sensitive.
Another way to address this that I've adopted as my default approach is:
Open the old file (using the old version)
set the default/guest account to full access
convert the file to new version
go into the new version file using the guest account which has full access
re-establish the full access account and guest account behaviors.
This approach took some of the worrying/sweating out of the process by avoiding the "The account name or password is invalid" notice. That always makes me paranoid...