Is it possible to move items (tasks, milestones, even multi-tiered projects) from one Aeon Timeline file to another?
Yes you can copy items between files, however it is limited and not everything is copied.
For example, relationships and dependencies are not copied, not are properties if they don’t exist in the other file’s template.
You also can only copy an item on a view that will contain it, therefore this will limit some items such as Characters, as in the default Fiction template they would only be able to be copied onto Mindmap View.
If you are wanting to move items between files, it is potentially better to do a csv import/export, which you can read about here: Import, Export & Printing - Aeon Timeline 3 Knowledge Base
This will give you more control over what is moved across.