$folders = @("D:\test", "J:\zbackup")

foreach ($folder in $folders) {

Get-ChildItem $folder -Recurse -File | ForEach-Object {

$old = $_.FullName
$name = $_.Name
$lower = $name.ToLower()

if ($lower -eq $name) { return }

$dir = $_.DirectoryName
$temp = Join-Path $dir ([guid]::NewGuid().ToString() + ".tmp")

Rename-Item -LiteralPath $old -NewName (Split-Path $temp -Leaf)
Rename-Item -LiteralPath $temp -NewName $lower

Write-Host "RENAMED: $name -> $lower"
}
}