If I import a keyword list into an existing set of keywords:
When you import a list of keywords using Metadata > Import Keywords, your existing keywords are always preserved, and new keywords in the imported list are added to LR's keyword list. The comparison uses the full hierarchical name of the keyword, e.g. Family > Smith > John is a different keyword from Family > Ellis > John. While you can set a new keyword's Include On Export attribute by enclosing it in square brackets, you can't change the attribute of existing keywords. The assignment of keywords to photos is left unchanged.
So you can't use Import Keywords to rearrange a hierarchy of existing keywords. If in the import list you try to "move" the leaf keyword Family > Smith > John to Family > Ellis > John, you'll end up with two keywords Family > Smith > John and Family > Ellis > John. The only way to change the hierarchy of existing keywords is via drag and drop in the Keyword List panel, or by adding a new keyword, manually reassigning the old photos to the new, and then deleting the old keyword.