How do I change from level to level. Say I have a level with a door in it. Player opens the door and walks in - What script do I use to move the player from level_1 to level_2 by using the door?

Any help would be appreciated.

There are many diffrent ways but the most common is to place an entity that will serve as trigger , when the player collides with it (with ent_move/c_move , event_entity or event_impact is triggered) it executes 'load_level' function (or was it level_load?)
I'd do it using two vars , if a player.x is more than , and less than (same for y) load_level...
Check in the AUM , I think there was something about changing levels + carrying your items (using the templates) with you.

in the templates, there is the level change script, but the script only allows you to change levels 9 times, but i use a script for the au resources, that allows for 100 levels, it works great! you just have to include it in your scripts.