Here is the secret that Fortinet support doesn't shout from the rooftops: You don't need a support contract to access old versions, but you do need a free account. forticlient vpn download older versions

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WARNING: Proceed with extreme caution. Here is why: WARNING: Proceed with extreme caution

Once you have safely downloaded your desired older version (e.g., FortiClientSetup_6.4.9_x64.exe), follow these steps to avoid conflicts.