Modify list item field using PowerShell

I have a list field that stores an XML string.

I am trying to get the XML contents and modify a particular tag in the XML like so:

foreach($item in $list.Items)
{
    if($item["XMLField"].Contains("NodeImLookingFor"))
    {
        $itemNum = $item["ID"]
        Write-Host ("{0} - {1}" -f $s, $itemNum)
        $a = $item["XMLField"]
        $a = $a.Replace("NodeImLookingFor", "ReplacementNode")
        $item["XMLField"] = $a
        $item.Update()
    }
}

The script above, however, deletes the contents in that field. Frustrated, I started over and repopulated the field with the XML. Once repopulated I tried the following:

foreach($item in $list.Items)
{
    if($item["XMLField"].Contains("NodeImLookingFor"))
    {
        Write-Host $item["XMLField"] #Displays the XML content in the cmd window
        $a = $item["XMLField"]
        Write-Host $a   #Does not display anything
    }
}

Now i'm confused should I just run the replace command straight on $item["XMLField"] instead of sticking its contents into the $a variable and editing it from there. My concern is I will delete the list field contents regardless. Please advise.