Change upn-suffix for all the users in certain OU

Get-ADUser -Filter {UserPrincipalName -like "*@domain.local"} -SearchBase "OU=whateverisright,DC=contoso,DC=com" |

ForEach-Object {

$UPN = $_.UserPrincipalName.Replace("domain.local","newdomain.local")

Set-ADUser $_ -UserPrincipalName $UPN

}