Is it possible to alter terrain once proper trigger conditions are met? Im using Staredit.

Nope. Neither terrain nor doodads can be affected via triggers.

Except for installation traps

Yeah, even with EUD's I think its highly unlikely that you can modify terrain using triggers... It would be cool though. Walk to an area and have a huge bolder turn into a passage way... *Dreams On*

If you've played "House of the Haunted" by (U)Bolt_Head you'll notice that a very similar effect can be archieved by clonning a chunk of terrain and then using properly the move unit and center view trigger actions

The elevator scheme?

Yeah exactly.
Also some other RPG used center view to sumilate an earthquake and then moved the players to another part of the map that was exactly the same but had the bridge broken. Don't remember which RPG had it though.

Yeah, those are the maps that really separate themselves from the rest of them...

Only problem with that method is if you even glance at the minimap you can see the effect is obvious It looks good otherwise though.

You can't change terrain in the game but in scmdraft and starforge you can place some of the sprites (doodads basically) from other tilesets onto any tileset. Some useful ones are rocks and trees.

Of course you can do that. He said in-game though. Here's an idea though. Create a bunch of buildings (terran for rubble, protoss for another color of rubble, and zerg for bloody, something) and destroy them. They leave the corpse/rubble on the ground for a while. Most sc players know that. Pseudo terrain change? Hide actual terrain with that, and after the a while, all that stuff disappears. You could have an area where there was an earthquake, (kill and destroy the terran buildings) then have a peon (or whatever you use as a worker in your map) running over it, so it looks like they're removing it. After a while, it all disappears.