$spWeb = Get-SPWeb("http://abc.com/corp/tis/TISWorkspaces/Toxicologyproject/")
$spFile = $spWeb.GetFile("http://abc.com/corp/tis/TISWorkspaces/Toxicologyproject/Pages/Home.aspx")
$spFile.CheckOut("Online",$null)
$spFile.Properties["PublishingPageLayout"] = "/corp/_catalogs/masterpage/DepartmentPageLayout.aspx, Department Page Layout"
$spFile.Update()
$spFile.CheckIn("Update page layout via PowerShell",[Microsoft.SharePoint.SPCheckinType]::MajorCheckIn)
$spWeb.Dispose()
From: http://osric.com/chris/accidental-developer/2011/11/updating-a-sharepoint-pages-page-layout-using-powershell/