This entity causes a level transition, where a new map is loaded and the player is placed in it.
Name (targetname) - Property used to identify entities.
New map name (map) - The name of the new map to load.
Landmark name (landmark) - The name of the info_landmark that defines the same point in space in both maps.
Change Target (changetarget) - The entity name to be triggered in the next map. Works similar to a trigger_auto.
Delay before change target (changedelay) - Self-explanatory.
No Intermission (1) - This flag appears to be unused and has no effect.
USE Only (2) - Entity can only be triggered by another event.
The game must be restarted completely if updates to the level change brushes are made. Not doing so causes incorrect and/or problematic transitions, even if all the change level brushes and entities are properly set up and configured.
It's recommended to keep your map names short with no capital letters, spaces, or special characters. Instead of "Chapter 3 Level 5", try c3l5 or ch3_lvl5.
It's recommended to follow the same naming conventions for landmarks - no capital letters or spaces / special characters.
You must set up changelevels between two maps both ways viz. map1↔map2, even if the player is meant to progress one way. Where players aren't meant to backtrack, place the "return" trigger as a small brush in an unreachable place, with flag 2 (use only) set. Not setting this up properly will crash the game.
If a trigger_transition is set up with the same name as the info_landmark, the trigger_changelevel can be triggered by other entities. The player will change level if they are inside the trigger_transition when the trigger_changelevel gets triggered. This also meant that the trigger_changelevel can be placed anywhere on the map; the trigger_transition takes care of the affected area.