PowerShell XML: Switch vs if

Doing a simple find/replace with an XML file via PowerShell. I am getting errors when attempting to replace text in a node but only when using a switch...using a general if statement works perfectly.

Code starts thusly:

$oldString = 'AAAA'    
$newString = 'BBBB' 
[xml]$xml = Get-Content $myXMLFile
$list = Select-Xml -Xml $xml -XPath '//add'
$list|Foreach-Object {
   [string]$key = $_.node.key
   [string]$value = $_.node.value

The plain old IF version works as I would expect...

   if($value -eq $oldString)
   {
      $_.node.value = $newString
   }
}|Set-Content $myXMLFile

But replacing the IF with a switch statement throws an error.

   switch($value)       
   {
       $oldString{$_.node.value = $newString}
   }    
}|Set-Content $myXMLFile

Property 'value' cannot be found on this object; make sure it exists and is settable.
+          $oldString{$_.node.value = $newString}
+                    ~~~~~~~~~~~~~~~~~~~~~~~~~~~
     + CategoryInfo          : InvalidOperation: (:) [], RuntimeException
     + FullyQualifiedErrorId : PropertyNotFound

I have a lot of node data to replace and I would rather not write a bunch of IF's if I can get away with a switch.