"if you want to rename a runtime, it will not be recognised. You have to put it inside a folder called "old stuff", but the names runtime and its subfolders must be kept."

I've found you CAN rename it and it'll be recognized -- but only if you keep the word "runtime" in the new name. My oldest one is called "Runtime1".  I also have a "Test Runtime" and a "Personal Use Only Runtime".  Told Poser where to find 'em & they worked just dandy.