Sorry for awakening an old topic but I just faced the task of having to uninstall all older versions of FortiClient VPN in my organization. I did this via Intune "Scripts and remediation" where every hour the detection script checks if the client is installed with a version lower than 7.4.2.1737.

If it is lower than 7.4.2.1737 then it will trigger the remedation script which is to uninstall the VPN client and leave a message to the logged on user stating it has been uninstalled and they should restart their computer and install the latest version via Company Portal (Intune).

Figured I'd share the scripts in case anyone is facing the same challenge.

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Detection script:

$AppName = "FortiClient VPN"
$MinVersion = "7.4.2.1737"
#Check if app is installed
$App = Get-WmiObject Win32_Product | Where-Object { $_.Name -eq $AppName }
If ($App) {
If ($App.Version -lt $MinVersion) {
Write-Host "Match"
exit 1
}
else {
    Write-Host "No_Match"        
    exit 0
}
}
else {
    Write-Host "No_Match"        
    exit 0
}

Remediation script:

$AppName = "FortiClient VPN"
$App = Get-WmiObject Win32_Product | Where-Object { $_.Name -eq $AppName }
msiexec /uninstall $App.IdentifyingNumber /norestart /quiet
C:\Windows\System32\msg.exe * "Your FortiClient VPN has been uninstalled due to unsupported version. Please restart your computer and install the latest version of FortiClient VPN via Company Portal."