Unfortunately there is no way to change the default library - ADE will only look or the folder in your My Documents directory.
You can move the files from the My Digital Editions folders afterwards and then add them back in to your library.
You can specify the library location in the XML Manifest file....
You can specify the file location in the manifest file, the location of the manifest file itself (which is the library) is fixed. As is the location that ADE downloads files to.