[Kazun]

Код:

$Res =  Switch ($Surname.Substring($Surname.Length-2)){  
                            "ец" {($Surname[0..($Surname.Length-3)] -join "") +  "ца"}
                            ("зе", "их", "ых" -eq $_)  {$Surname}
                            "ой" {($Surname[0..($Surname.Length-3)] -join "") + "ого"}
                            "уй" {($Surname[0..($Surname.Length-3)] -join "") + "уя"}
 }