Illustrator Tip #856t292: Copying complicated stuff between documents.
A lot of the time, when you try to copy from one document and paste into another, Illustrator will decide to expand complex appearance stacks and bitmap effects into something completely uneditable. You can get around this by opening each document in a separate window and dragging the objects from one document to the other.
Thankfully, this will respect Paste Remembers Layers if you’re copying really complicated stuff spread out over multiple layers.
It will not copy over any Graphic Styles you may have used. I’m not sure if that is preferable to AI’s tendency to create duplicates of Graphic Styles when cutting and pasting stuff around