[Microsoft.SharePoint.SPSecurity]::RunWithElevatedPrivileges({
param([string]$url = "http://sharepointsite/", `
[string]$oldprovider = `
"keystoneuser", `
[string]$newprovider = `
"i:0ǵ.t|keystone sts|radiant\")

add-pssnapin microsoft.sharepoint.powershell -EA 0

# Get all of the users in a site
#$users = get-spuser -web $url -Limit "11000"
$csvFile = "C:\Users\Users.csv"
# Loop through each of the users in the site
Import-Csv $csvFile | ForEach-Object{
# Create an array that will be used to split the user name from the domain/membership provider
$a=@()
$username = $_.oldlogin
$a = $username.split(":")
$userloginprovider = $a[0]
$displayname = $a[1]
Write-Host "Old Username: $username."
# Create the new username based on the given input
$newalias = $newprovider + $displayname
$user = Get-SPUser -Identity $username -Web http://sharepointsite/ -EA "SilentlyContinue"
if($user -notlike "")
{
Move-SPUser –Identity $user -NewAlias "$newalias" -IgnoreSID -Confirm:$false -EA "SilentlyContinue"
Write-Host "User Changed to $newalias"
}
}
}); 



To migrate user in farm use the following,


$farm.MigrateUserAccount( $_.oldlogin, $_.newlogin, $false )

$site = Get-SPSite http://peakfinder/sites/site
$web = $site.RootWeb
  (or $web = $site.OpenWeb("site")
$folder = $web.RootFolder
$folder.WelcomePage = "SitePages/home.aspx"
  (or  $folder.WelcomePage = "default.aspx")
  (or  $folder.WelcomePage = "SiteAssets/myhome.aspx")
$folder.update()
$web.Dispose()
$site.Dispose()