Configure suas variáveis
$Filepath = 'C:\Users\mad tom vane\Documents\Test1.xlsx'
$Filepath = Get-Item -Path $Filepath
$NewFilepath = Join-Path -path $Filepath.directory.fullname -ChildPath "$($Filepath.basename).xls"
$Excel = New-Object -ComObject Excel.Application
$Excel.Visible = $true #or false
Eu gosto de abrir em somente leitura
$Workbook = $Excel.Workbooks.Open($Filepath.fullname,[Type]::Missing,$true)
Salve a pasta de trabalho como xlExcel8 para um arquivo XLS
$Workbook.SaveAs($NewFilepath,56)
#https://technet.microsoft.com/en-us/library/ff730962.aspx
$Workbook.Close()
$Excel.Quit()
[System.Runtime.Interopservices.Marshal]::ReleaseComObject($Excel)
Remove-Variable Excel