There are two ways to approach reorganizing your descriptive hierarchies in the user interface.
The first is the drag and drop options supported in the treeview. This can be used to reorder sibling records (for example, files in a series, etc). You cannot drag a description to a new parent, but if one of the siblings has child descriptions (for example, items under one of the file-level records in your series) then they will stay associated with the parent as you reorder. See:
The second option, which might be more applicable for your needs, is the move module. This allows you to move descriptions to any other location - include as a new top-level description, or as a child of another description. If the record being moved has children (for example, the series whose files we were reorganizing in the first example), then they will be moved as well and will retain their current associations. See:
Unfortunately, we don't have an easy way to do the reverse in bulk - select all the sublevels and move them all at once to a new parent, so you would have to do that one by one for your lower level descriptions if performing this via the user interface.
If neither of these methods seem practical for your needs (for example, you need to move thousands of children to a new parent), then there may be some further options we can explore using SQL. Let me know if you'd like me to explore this option further, and I can provide some example queries.