Upgrade a Windows Edition with a MAK Key from PowerShell

<#
.SYNOPSIS
    Installs a Windows product key (for example an Enterprise MAK) and activates it, logging each step.
.DESCRIPTION
    Uses the Software Licensing WMI classes, the same ones slmgr.vbs calls, to install a product key,
    activate Windows online, and confirm the result. If the key belongs to a different edition that
    supports a key-based upgrade (Pro or Education to Enterprise, for instance), Windows switches
    edition; a restart finishes the job. The key is taken as a SecureString and never written to the log.
    Run it elevated. Supports -WhatIf.
.PARAMETER ProductKey
    The product key, as a SecureString. Prompted for if you leave it off.
.PARAMETER ExpectedEdition
    The EditionID you expect afterwards, for example Enterprise. Used for the check at the end.
.PARAMETER LogPath
    Where to write the log. Default: C:\Windows\Temp\Update-WindowsEdition.log
.PARAMETER Restart
    Restart the computer when everything succeeded. Off by default.
.EXAMPLE
    .\Update-WindowsEdition.ps1 -ExpectedEdition Enterprise -WhatIf
.EXAMPLE
    .\Update-WindowsEdition.ps1 -ProductKey (Read-Host -AsSecureString 'Product key') -ExpectedEdition Enterprise -Restart
#>
#Requires -RunAsAdministrator
[CmdletBinding(SupportsShouldProcess)]
param(
    [securestring]$ProductKey,
    [ValidateNotNullOrEmpty()][string]$ExpectedEdition = 'Enterprise',
    [string]$LogPath = (Join-Path $env:SystemRoot 'Temp\Update-WindowsEdition.log'),
    [switch]$Restart
)

$ErrorActionPreference = 'Stop'
$windowsAppId = '55c92734-d682-4d71-983e-d6ec3f16059f'   # ApplicationID for Windows itself in SoftwareLicensingProduct

function Write-Log {
    param([string]$Message)
    $line = '{0:yyyy-MM-dd HH:mm:ss}  {1}' -f (Get-Date), $Message
    Write-Verbose $line
    Add-Content -Path $LogPath -Value $line -WhatIf:$false
}

function Get-WindowsLicense {
    Get-CimInstance -ClassName SoftwareLicensingProduct -Filter "ApplicationID='$windowsAppId' AND PartialProductKey IS NOT NULL" |
        Select-Object -First 1
}

$editionKey = 'HKLM:\SOFTWARE\Microsoft\Windows NT\CurrentVersion'
$before = (Get-ItemProperty -Path $editionKey).EditionID
Write-Log "Starting. Current edition: $before"

if (-not $ProductKey) { $ProductKey = Read-Host -AsSecureString 'Product key' }
$plainKey = [System.Net.NetworkCredential]::new('', $ProductKey).Password.Trim().ToUpper()
if ($plainKey -notmatch '^([A-Z0-9]{5}-){4}[A-Z0-9]{5}$') { throw 'That doesn''t look like a product key (XXXXX-XXXXX-XXXXX-XXXXX-XXXXX).' }
$lastFive = $plainKey.Substring(24)

$result = [ordered]@{ ComputerName = $env:COMPUTERNAME; EditionBefore = $before; EditionAfter = $null; KeyEndsWith = $lastFive; Activated = $false; RestartNeeded = $false }

if (-not $PSCmdlet.ShouldProcess($env:COMPUTERNAME, "Install product key ending $lastFive and activate")) {
    return [pscustomobject]$result
}

try {
    $service = Get-CimInstance -ClassName SoftwareLicensingService
    Write-Log "Installing product key ending $lastFive"
    Invoke-CimMethod -InputObject $service -MethodName InstallProductKey -Arguments @{ ProductKey = $plainKey } | Out-Null
    Invoke-CimMethod -InputObject $service -MethodName RefreshLicenseStatus | Out-Null

    $license = Get-WindowsLicense
    Write-Log "Activating $($license.Name)"
    Invoke-CimMethod -InputObject $license -MethodName Activate | Out-Null

    $license = Get-WindowsLicense
    $result.Activated = ($license.LicenseStatus -eq 1)
    Write-Log "License status: $($license.LicenseStatus) (1 = licensed). Channel: $($license.ProductKeyChannel)"
}
catch {
    Write-Log "FAILED: $($_.Exception.Message)"
    throw
}
finally {
    $plainKey = $null
}

$after = (Get-ItemProperty -Path $editionKey).EditionID
$result.EditionAfter  = $after
$result.RestartNeeded = ($after -ne $before)
Write-Log "Edition now reports: $after"

if ($after -ne $ExpectedEdition) {
    Write-Warning "Edition is '$after', expected '$ExpectedEdition'. Some changes only show after a restart."
    $result.RestartNeeded = $true
}

[pscustomobject]$result

if ($Restart -and $result.Activated -and $PSCmdlet.ShouldProcess($env:COMPUTERNAME, 'Restart')) {
    Write-Log 'Restarting'
    Restart-Computer -Force
}