What I do for level changes is this..
In say
level1.rmf I set up the trigger_changelevel, and info_landmark
Then once done, I click 'Save As' and save the map with a different name:
level2.rmf Now, this map is the same as 'level1.rmf' just with a different name.. now all you have to do it change the 'Trigger_changelevel's' map name back to map1.. and you got your Change level..
So delete the unwanted parts.. (the already covered parts) and add on your next part.. this way you'll know, where ever your landmark is, it will be in the same place as both maps.