BranoI hate VogonsPremium,MVMReviews:
All ZyXel upgrades are full upgrades, you can jump directly to latest version available.
Just make sure you backup your startup-config.conf prior the upgrade just in case something goes wrong.
No need to upgrade bootbase (actually you can't upgrade it), ZyXel is bundling bootbase upgrades into normal upgrade if necessary to upgrade bootbase.
You can get all your firmwares and manuals from here: »ftp://ftp.zyxel.com/ZYWALL_USG_200/