there should be a
Trigger_ChangeTarget if you've added the generic HL enitiy list.
In Hammer 3.x go to Tools-> Options
Under Game Data Files, you should have the NS.FGD, you need to add the HalfLife.FGD as well. (If i remember correctly) It also comes with the Hammer install, under the c:\path_to_hammer_install\fgd\half-life....
otherwise, there is another way to do it, but it may cost you about 5-6 entities.