* Can you use the scenario editor to change the campaign missions?

I wanted to add a few hero units in the final mission "All In". It just makes sense that key heros would be on the field (Raynor, Tychus, etc.) If it is possible, does it disable the achievements? I tried to add units to the "All In" map, but it game me a notice telling me that I violated all sorts of triggers or something like that. Any advice? Thanx.

 

For those of us who haven't finished the campaign yet, please label your post with a SPOILER ALERT! Thanks.

You would be best changing the mission and playing it on your own rather than through the campaign, because I am sure you won't be able to get it to change and still have achievements/work in the campaign section of the game/etc.