Update-XmlByTransform.ps1

<#
.SYNOPSIS
    Uses XML transformation on a specified XML file
.DESCRIPTION
    Performs XML configuration changes to a given XML file via XML-Document-Transform (XDT) commands specified within a seperate XML file.
.EXAMPLE
    PS C:\> Use-XMLTransform -XML 'D:\myApp\app.config' -XDT 'D:\myApp\app.release.config'
    Transforms configuration within app.config using the XDT commands found in app.release.config
.INPUTS
    String literal paths to XML/XDT docs and optionally XmlTransform.dll
.OUTPUTS
    boolean - true if successful. false if unsuccessful
.NOTES
    Web.config Transformation Syntax - https://msdn.microsoft.com/en-us/library/dd465326(v=vs.110).aspx
    Microsoft ASP.NET and Web Tools - https://marketplace.visualstudio.com/items?itemName=JacquesEloff.MicrosoftASPNETandWebTools-9689
#>
function Update-XmlByTransform {
    [CmdletBinding(SupportsShouldProcess, ConfirmImpact = "Low")]
    param(
        # Path to the xml file being transformed
        [Alias('XML', 'Path')]
        [Parameter(Position = 0, Mandatory, ValueFromPipeline)]
        [ValidateScript(
            { Test-Path $_ -PathType 'Leaf' }
        )]
        [string]
        $FilePath,

        # Path to the transform file
        [Alias('XDT', 'TransformFile', 'TransformPath')]
        [Parameter(Position = 1, Mandatory)]
        [ValidateScript(
            { Test-Path $_ -PathType 'Leaf' }
        )]
        [string]
        $XmlTransformFile,

        # Path to the XML transform assembly. Comes with Microsoft ASP.NET and Web Tools https://marketplace.visualstudio.com/items?itemName=JacquesEloff.MicrosoftASPNETandWebTools-9689
        [Alias('DllPath')]
        [Parameter(Position = 2)]
        [ValidateScript(
            { Test-Path $_ -PathType 'Leaf' }
        )]
        [string]
        $XmlTransformDllPath = 'C:\Program Files (x86)\MSBuild\Microsoft\VisualStudio\v14.0\Web\Microsoft.Web.XmlTransform.dll'
    )
    begin {
        $result = $false
        Write-Verbose "Adding XmlTransformableDocument type from DLL at '$XmlTransformDllPath'"
        Add-Type -LiteralPath $XmlTransformDllPath

        Write-Verbose "Loading Xml Transform Document from '$XmlTransformFile'."
        try {
            $TransformDocument = New-Object Microsoft.Web.XmlTransform.XmlTransformation($XmlTransformFile)
        }
        catch {
            Write-Output $result
            Write-Error "Transformation file load failed. The error encountered was: $($_.Exception)" -ErrorAction Stop
        }
    }
    process {
        Write-Verbose "Loading Xml File from '$FilePath'."
        $XmlDocument = New-Object Microsoft.Web.XmlTransform.XmlTransformableDocument
        $XmlDocument.PreserveWhitespace = $true

        try {
            $XmlDocument.Load($FilePath)
        }
        catch {
            Write-Output $result
            Write-Error "XML file load failed. The error encountered was: $($_.Exception)" -ErrorAction Stop
        }

        try {
            Write-Verbose 'Applying transform.'
            if ($PSCmdlet.ShouldProcess($FilePath, "XML Transform using '$XmlTransformFile'")) {
                $result = $TransformDocument.Apply($XmlDocument)
                if ($result) {
                    Write-Verbose 'Saving changes'
                    $XmlDocument.Save($FilePath)
                }
            }
        }
        catch {
            Write-Error "Transformation failed. The error encountered was: $($_.Exception)"
            if ($_.Exception.InnerException) {
                Write-Error $_.Exception.InnerException
            }
        }
        finally
        {
            Write-Output $result
        }
    }
}