First off, FMP 5 files didn't have account names. When the file is converted, it'll ask you for a password the first time you open the file. Just enter the password twice in both the account name and password boxes and the file will open. Once you've got it open, you can go to Manage | Accounts & Privileges to change the account name so that other folks can't see what password is being typed in.
If the password truly was changed, I'd search the web to see if you can find a password cracking utility for that version of FMP 5. Such a widget did exist once and you can then use it and and a copy of your FMP 5 file to figure out the new password. Then discard the hacked copy of your file and convert the original to FMP 10.