If you are looking to upgrade your FortiGate to the latest version of code you need to stop and read the links listed below. There is a supported upgrade path for each version of FortiOS (4, 5, 5.2, 5.4, etc). If you want to progress through the versions of code without nuking parts of your config or causing yourself to experience strange explainable behavior (that’s a trick question since it is definitely explainable….you didn’t follow the upgrade path!) then you please follow these instructions.
Click the link below and scroll down to the “The Upgrade Path Tables”. This page will tell you how to get to the version of code that you wish in a safe and secure manner!