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Well, that's exactly what I was doing; I was copying the poster and then modifying the copy, essentially creating a new one.
But here's another problem that arose, and it's unclear why.
I created a folder in AdvMapStaticShared, and inside it, I created a copy of the poster. I added a dead creature to the map and did this with several different dead creatures, saved, and exited.
Today, I open the editor and realize that it doesn't recognize the new folder, along with everything inside it. It only shows the shared dead creature if you click on it, but there's only the object for that one, and not for the others.
I don't understand what the problem is at all. If I unzip the map, all the objects are there.