Keep in mind that the OS upgrade step is the tricky part.
Upgrade from 7.5.7 -> 7.6.5 is straight forward
Then you need to take a backup and pull the backup off the server
Reinstall fresh from the 7.6.5 ISO
Restore the backup
Then upgrade to 7.7.12
8.0.3 should be available soon, it's still in testing.