Passwords are case sensitive in .fp7 format , so if you have two files in your .fp5 solution that had full access password "MyPass" for the first file and "mypass" for the second both files would open in filemaker up to 6 nomatter what you typed ,MyPass or mypass.
After the conversion the first file will have an account created "MyPass" and a password "MyPass"
but the second file will have account "mypass" and password "mypass",
so you must enter "MyPass" account and password for the first file,
and "mypass" account and password for the second.
Change your password in both files to the same case before converting.