What's the trigger in the "Mercenaries" map that switches a specific unit out for a hero when its built?

I have no idea what Mercenaries is, but if it functions anything like Diplomacy, all you do is create the hero in some area, center a location onto the built unit, and then remove that unit and move the hero there. You want to create the hero and then move it so that you don't get the "Unit Implaceable" message. So basically, the trigger looks like this:

I see. Thanks, that should help.

Here's how Blizzard did it (they didn't protect their maps )

which doesn't require an extra "spawning zone" but could get an implaceable error if you, say, replace units with units of larger size.

You definitely want the extra spawning zone. There are numerous ways you can cause an unplaceable unit error, even when replacing a unit with a smaller unit.

Indeed; it's best practice to always create units this way. This way it's always ensured that your unit can be placed and will be created.

I look forward to using this in a campaign map.