Hello,
You can upgrade to version 10.0.0-7 directly from 8.0.3 version. You can do that two ways, one of that is from Web Interface and the other one from CLI.
I prefer to use second one because you couldn't see any progress status about upgrade before 9.x versions.
1- Administration/Configuration/Version/, you can see that 10.0.0-7 version is available for update. You can select the version and then click upgrade. Upgrade might take a couple of hours(first downloading the newest version then installing it automaticaly)
2-Just login to appliance from SSH with "Admin" user then use "update check" (check the versions) and after it completed, use "update download" for downloading the newest version to your appliance then use "update install" after downloading. You can see the progress when downloading and installing and also you can install anytime you want once you downloaded files.
Before all of these, create a full backup of your system and download it to your local and keep the ISO file of 8.0.3 for emergency to restore your backup if available.
Regards,
Oykun