When you are upgrading FortiGate firewalls, no doubt you have been using the little "Update" button on the dashboard and the built-in GUI. What you may not realize is that you cannot just upgrade to the latest version without checking if it is compatible!
The GUI firmware upgrade in FortiGate firewalls simply checks for the latest firmware for your hardware, it does not verify that the upgrade path is compatible. You must first check the release notes and verify that the firmware you are running is a supported upgrade path.
How to verify
To verify the firmware you have is compatible, first go to docs.fortinet.com/fortigate/release-information and choose the release notes for the version you are trying to install. Then scroll down to "Update Information" in the table of contents, and click that to go directly to that page. You should see a section like this:
Note that it lists the compatible versions (highlighted in red). If you aren't upgrading from one of those versions, you should check the older versions until you find one that you can upgrade to. Unfortunately as of this writing, FortiGate doesn't automatically determine the newest version you can upgrade to.
What if I upgrade anyway?
You can always choose to upgrade to the latest version anyway, however you may find that all of your settings did not migrate correctly. As an example, I ran into a scenario where SNMP settings did not migrate correctly.
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