Thanks for posting!
The default username and password for files created in FileMaker Pro 7 through Filemaker Pro 11 is Admin and no password.
Since the account structure changed between FileMaker Pro 6 and FileMaker Pro 7, we have a couple of possibilities as far as what your current password is. If you've never had a password on the file in FileMaker Pro 6, then, after conversion, the Guest account should be enabled and set to Full Access. If you did have a password set, the conversion process should take that password and set it to be both the username and password of the new file.
For more information on the security changes that may occur to the file after conversion, please see knowledge base answer #571:
You'll need to check your converted files for other issues after conversion once you resolve the password issues. http://help.filemaker.com/app/answers/detail/a_id/750/kw/converting
The conversion wen pretty well for me when I converted a similar sized set of 5.5 files, but there were some niggling little details that had to be fixed, so be prepared to do some extensive testing to make sure you've tied up all the loose ends here.