xmlAttributesFindReplace.ps1

[xml] $x = '<Lunch>
<Sandwiches>
<Sandwich Include="Mixte">
    <Filling>Ham and Emmantal Cheese</Filling>
</Sandwich>
<Sandwich Include="RoteDeBoeuf">
<Filling>Beef and Mustard Mayonnaise</Filling>
</Sandwich>
</Sandwiches>
</Lunch>'

$Mixte = "Fromage au jambon et Emmantal"
$RoteDeBoeuf = "Mayonnaise au boeuf et à la moutarde"
$keys = $($x.Lunch.Sandwiches.Sandwich)
foreach ($var in $keys) {
    $update = $var.Include
    if (Test-Path variable:$update) {
        [string]$value = Get-Variable $update -ValueOnly
        Write-Verbose ('Setting variable: {0} = {1}' -f $update, $value) -Verbose
        $element = $x.SelectNodes("/Lunch/Sandwiches/Sandwich") | Where-Object {$_.Include -eq $update} 
        $element.Filling = $value
    }
    else {
        $missingVariables += $var
    }
}
if ($missingVariables.Count -gt 0) {
    throw ('The following variables are not defined in the current scope (but are defined in the xml): {0}' -f ($missingVariables -join " `n"))
}

$x.Lunch.Sandwiches.Sandwich