You don't need a trigger once, because once you go to the layout it will shift to the new layout event sheet, so, it won't probably be checking for the conditions, unless you use one event sheet for both layouts, you won't need it.
Go to layout is a System action, you'll find it inside the System folder. There's one action with a droplist with all the layouts so you pick one or you can use Go to layout (by name) and type it in the box.